Yes, amazingly enough, there are rules for that. You'll need 195,000 GP (or half that if you're doing it yourself), and access to someone with Heal 10, the Graft Flesh Feat, the Disintegrate Spell (presuming that's the eye stalk you want to implant, and 195 days. ... Or you can decide to not bother.
Traditional, Bad, Mad Science
Looking at the grafts handbook, there are rules for "a crown of eyes", "eye stalks", and "replacement eyes" which can be found in the original Fiend Folio page 207. All of which require absurd amounts of gold and time to implant (the DM may give you a discount if you provide the original eyes, rather than having to source them through your HMO). By the rules, the "flesh of the original creature" is a necessary material component, and so the eye stalk will run you a total of 195,000 gp.
Instead, consider using this (with DM approval -- and I only suggest this considering the nature of your game as first described) as the basis of progression into an "Beholder Mage" from Lords of Madness (with the in-game fiction of grafting all the beholder's eye stalks to your head). Nominally, this class is overpowered (due to a 1 spell level per level progression), but should integrate neatly with your theurge progression as it stands, without too much "over-" or "under-" poweredness.
A proper evil death glare.
Your character is a Sorc 4/Cleric 3/MT 5. For our purposes, this means that you're a Sorc 9/Cleric 8 ... which is absolutely horrible for our purposes. I'll presume that your DM will allow retraining under the thematic imprimatur of "getting a beholder eye implanted."
Here, we have the objective of "firing disintegration rays from your eyes at level 12 as a spontaneous caster" with a side bonus of "firing combustion rays from your eyes when you feel like being more effective." (Combust, Spell Compendium, Sor/Wis 2 is one of the best spells to power through ocular spell) (It also means you can glare at someone so hard that they burst into flames.)
The first option is to posit that your god is really offended by your grafting of aberrations into you (for some odd reason...) and that your cleric levels desert you. This leaves you as a simple sorcerer/12 (which is bad, but not as bad as your current build.) As a consequence of this "retraining" you'd take the metamagic feats: Ocular Spell, Chain Spell, Practical Metamagic (Ocular Spell), Practical Metamagic (Chain Spell), Arcane Thesis (Disintigrate). With Arcane Thesis and Practical Metamagic, you get Ocular Spell for free on disintegrate. With the chain metamagic, Combust (Sor Wis 2/+1 Ocular/+2 chain = 5) allows you to store "Evil Fiery Death Glare" in your eyes which can chain target 13 creatures. (10d8 primary damage + ignition, and 12 targets worth of 5d8 damage + ignition).
You can do a similar thing with cleric, divine metamagic, and a whole bunch of nightsticks, as the "aberration taint drives out your inherent magical abilities." You would have to change one of your domains to Entropy, but your new eye would like that. Oh yes it would. Yes... ::cough:: If you can advise us to your favourite sorcerer spells, there's likely a domain or some other combination of capabilities which would provide for them.
Other builds would be significantly more cheesy, and would require significant discussion with your DM to account for your suddenly increased power level.
A little more out there
Looking at this from a different perspective, "can I have my level 12 character replicate a beholder's eyes?" ...
For the canonical beholder, we have:
disintegrate objects, transmute flesh to stone, cause sleep, slow the motion of objects or beings, charm animals, charm humans, cause death, induce fear, levitate objects, and inflict serious wounds
Charm Monster: single target save or lose
charm person: single target save or lose
Disintegrate: single target 2d6/Cl damage.
Fear: single target save or suck
Finger of death: single target save or die
flesh to stone: single target save or lose
inflict: hahaha. ignore.
Slow: single target save or suck
Cause Sleep: single target save or lose, any amount of HD
Looking at the list above, a spellcaster binder multiclass can achieve the specific eye rays a few times a day, and can use strategic binds to at least simulate the effects.
In this instance, I'd propose a complete rebuild to binder 1/wizard 3/Anima Mage 8 (Wizard is important since that's the only way to get access to level 6 spells here. If you've gained a level in the meantime, you are welcome to return to sorcerer if you must, though the progression will suffer.)
With Binder 9, (and improved binding, you can treat yourself like a Binder 11 for purposes of binding vestiges, allowing you to bind 2 level 5 vestiges.)
First, bind Geryon, as you get extra eyes in your head (located... behind each ear?) and an AoE Gaze attack that simulates the same "group take-down" capabilities of the Beholder (at significantly less power, but...) Secondly, bind Primus (Dragon 341) which allows you to issue an improved Command every 5 rounds, simulating the battle-field charm/sleep/fear rays.
By using vestige metamagic you can get two free ocular spells on flesh to stone and disintegrate. (And you can use the rest of your 6th level slots on other ocular metamagiced spells.) The only real trick here is that ocular spell prevents loading more than 2 spells into your eyes, even if you have more eyes. So this sort of thing is only the thing you can pull off at the start of combat. Still, with an at-will (every 5 rounds, but still) command (that also has some other cute benefits to your damage against chaotic creatures) and an gaze of acid, this allows you to pull off "being a beholder" for quite a few rounds a day. With this, it's also worth asking if your DM will houserule the eye graft above to (a slightly cheaper cost) to being able to hold ocular spells.
The ocular mailman: a fully cheesed Beholder Wannabe
Stepping further away from your character here, this is a direct-damage sorcerer build themed after a beholder. We're straying rather far from a simple rebuild here, but the arts of aberrations can be subtle indeed.
Here, we're focused not so much on the specific spells of the beholder, so much as the "here's a beam of direct damage", "here's a beam of save-or-lose", "here's a beam of join my team."
The direct damage beam will be combust (Spell Compendium). The save or lose beam will be Shivering Touch (Frostburn). And "join my team" will be accomplished with an ocular necrotic cyst. I'm going to go complete cheese here just to see how far we can get with the "beholder wannabe."
The feats needed for this will be:
- Ocular Spell
- Necrotic Cyst (worst case, you can dominate people into believing that you're a beholder.)
- Undead Battery (this feat is so cheesy. I would absolutely forbid it. Normally I'd recommend divine metamagic or incantrix, so it's not like this is novel)
- Prereq Greater Spell Focus (Necromancy),
- Spell Focus (Necromancy)
I'm going to build this around an ultimate magus chassis, both to power the metamagic when you don't have time to suck the undeath out of corpses you summon.
The Chassis I'm going to hang this on is Beguiler 1/Wizard 5/Spellthief 1/Unseen Seer 2/Ulitmate Magus 10/Paragnostic Apostle 1 (from here).
We'll need the spellthief guide to calculate caster level on this one. I'm going to avoid using the trickster spellthief simply because... I don't feel like digging up dragon magazine 353.
We use master spellthief to set all of our caster levels equal, so we keep boosting wizard as our "lowest" spellcasting level. On consideration, I like unseen seer 2 for hunter's eye. It means that you'll generally go around invisible, drop an illusion of a beholder (through whatever means) and sneak attack from within that illusion.
Critically, we get ultimate magus 3 by level 12
At level 12:
We're only a wizard 10 in terms of spells castable (remember, due to spellthief our wizard and beguiler caster levels are equal) and a Beguiler 4, most of which we'll be spending to power metamagic. You'll be using expanded spell knowledge to transfer "summon undead" over. At this level, you'll be sucking the undeath out of kobold zombies, but you'll graduate to bigger and better things soon enough. However, our caster level for these spells will be 10 (wizard)+4(Beguiler)+1(Spellthief)+1(Ultimate Magus) = 16. At level 12. (probably, I may have left out a modifier somewhere, I'll say it's 16 for now).
1: Spell Focus (Necromancy), Mother Cyst
1 bonus: Scribe Scroll
1: Flaw, whatever, Bonus: Twin Spell
1: Flaw, whatever, Bonus: Chain Spell
3: Greater Spell Focus (Necromancy)
6: Ocular Spell
6 Alternate Class Feature: Spontaneous Divination
9: Versatile Spellcaster (at this level, we need this to qualify for undead battery. I'd prefer twin spell but what can you do?)
12: Undead Battery (let the dice throwing commence)
As a benefit of versatile spellcaster, we can sacrifice two 5th level wizard spells for necrotic eruption, which is a save-or-die explosively, an absolutely worthwhile use of a feat. (How does sacrificing two divination spells to spontaneously cast a necromancy spell work? I have no idea. We're so deep into "DM throws dice territory that... it doesn't really matter.)
Anyway, as a CL 16 Wizard with 5th level spells and ocular spell, preparing for combat is entailed.
- Beguiler, summon kobold zombies with 9 total HD.
- Order them to allow you to eat their brains.
- Eat their brains (with ketchup).
- Presuming that you have a charisma modifier of +9, Prepare a twin chain ocular combust (using a level 2 wizard spell slot, or higher level spell slots as indicated by lower charisma modifier) into your left eye.
- Repeat for right eye.
Then, before combat, cast greater invisibility, major image (maintained through a skill trick of swift concentration) of a beholder mage, and flight. Glide serenely into combat, swift action hunter's eye, release your two twin, chain, ocular combusts sneak attacks as a full round action. Your primary target will take 40d8 fire damage + 20d6 sneak attack (assuming you hit with all four ranged touch attacks), be engulfed in flames, and then 16 secondary targets may all take up to 20d8 damage (and also be on fire.) It's likely that your group will want snacks prepared ahead of time (by you) for all the dice rolling here.
Presuming that some targets survived, cast an ocular necrotic cyst. Then, the next round, spontaneously cast necrotic eruption for 16d6 damage (on successful save) or a very very messy death. Feel free to use your major illusion to project a "evil ray" preceding the eruption.
It's quite likely that no-one will be around to critique your performance of a beholder after these casts. The only real trouble with this plan is that "reloading your eyes" takes a number of rounds, which isn't really the best plan ever in the middle of combat. For combats with a single target who isn't immune to cold damage (but you suspect not being cooperative with combust), prepare twin chain ocular shivering touches instead.