Getting shield itself
Items are, in general, a good source of magic that you don’t naturally get from your class. However, because shield is not on the bard spell list (and, somewhat surprisingly, no alternate class features for bard grants it), Use Magic Device may be necessary (but see the drake-helm option, below, for a case when it’s not).
Most likely, the item will require a DC 20 check: either you need to make up for your lack of the spell on your list (Emulate a class feature), or activate a wand or staff (Activate a spell-trigger item), which are coincidentally both DC 20. Between bards having that skill in class and bards using Charisma so heavily, even a 1st-level bard should succeed nearly half the time on that check. If he gets a +2 circumstance bonus from a masterwork tool for 50 gp, and a +5 competence bonus from a magic item for 2500 gp, he could get to the +19 bonus necessary always succeed on the check by around 5th level.
These are the most basic approach, and the only one available in core. You have basically three options:
Wand—potentially cheapest, but you want high caster level with shield, and that gets very expensive in a wand. Has the advantage of fitting in a wand chamber, an option in Dungeonscape allowing you to put one wand in a weapon or shield, so you can use shield without having to get out a big, cumbersome staff.
Costs caster_level × 750 gp.
Staff—uses your own stats, so you don’t need to worry about the item’s caster level, but it does have a minimum caster level of 8th. It also uses the same formula as wands, so if your wand of shield has caster level 8th or higher, a staff of shield is strictly superior for less money. Also ambiguous about whether or not single-spell staves are allowed. The good news is that spells after the first cost half as much; the bad news is, there probably aren’t many spells you really want to stick on the same staff.
Costs 6000 gp for the minimal caster level 8th version as long as you aren’t forced to include more spells.
Runestaff—from Magic Item Compendium, in addition to using your own stats like a staff, it also uses your own spell slots instead of charges. It is also dramatically cheaper than the other options, since it costs (spell_level)2 × 400 gp rather than spell_level × caster_level × 750 gp. A runestaff of shield costs just 400 gp. Like staves, runestaves “should have two to five arcane spells,” which may mean that you need to put a second one on there. But if you put another 1st-level spell in for 200 gp (like staves, runestaves charge half for spells after the first), that’s still a lot cheaper than the staff of shield.
Also have the drawback of limiting its use to 3/day for each spell. Many DMs just waive this since it’s already using your spell slots, but if yours doesn’t, see if the second spell can just be another 3/day uses of shield. 6/day will cover most adventuring days with fairly significant margin for error (also may cover all the 1st-level spell slots you have).
Costs 400 gp, 600 gp if you need to stick a second 1st-level spell in.
Each of these is going to require a UMD check every time you want to cast shield.
Knowstones from Dragon vol. 333 give you a spell known just for carrying them, and a knowstone of shield costs 1100 gp. The problem is that it only works for spells on your spell list: you would have to use Use Magic Device to get around that. This is a DC 20 check (Emulate a class feature), but it’s unclear when you make the check: do you make it to attune to the knowstone (which takes 24 hours), or every time you want to cast its spell?
Regardless of the answer, though, this is a better approach than any spell-trigger items. Yes, the runestaff of shield is cheaper, but you have to use the thing with both hands, while a knowstone of shield can sit in your pocket.
Costs 1100 gp.
From Explorer’s Handbook (a supplement aimed at the Eberron Campaign Setting), this helm has “sockets” for attuned Siberys dragonshards. These Siberys dragonshards basically work like rings of spell-storing, allowing you to cast a spell into one. But instead of allowing you to cast it out of the shard, when the shard is inserted into a drake-helm, the wearer of that helm can cast that spell with his own slots.
So as long as you can get someone to cast shield for you once, to put it in the shard, or you can just buy a shard that comes with shield already in it, you’re good to go.
Costs 1000 gp for 1-socket drake-helm and 6000 gp for a 1st-level attuned Siberys dragonshard. 7000 gp total. Does not require Use Magic Device checks.
In order, your best bet for getting shield are:
Sanctified spells can be prepared by any Good spellcaster. Yes, prepared: this requires the Arcane Spell Preparation feat from Complete Arcane for a bard to use. It’s worth it here.
Luminous armor and greater luminous armor are 2nd-level and 4th-level, respectively, versions of mage armor, granting +6 and +8 AC instead of +4. One key difference: they are abjurations. That makes these some of the best spells in the game for an abjurant champion.
By the way, bardic fighting
Bards can make excellent warrior-types. I strongly recommend you consider some things:
- Snowflake Wardance from Frostburn to allow you to add Charisma to attack rolls (with light or one-handed slashing weapons, and you cannot use a shield, so keep those restrictions in mind). You can also get the gauntlets of heartfelt blows from Magic Item Compendium to get fire damage equal to your Charisma bonus, and then at very-high levels, the slippers of battledancing from Dungeon Master’s Guide II allow you to swap Strength for Charisma on both attack and damage. All told, eventually that’s 2×Cha to both attack and damage, which is quite nice.
- Dragonfire Inspiration from Dragon Magic can dramatically amp up your damage, as it gives 1d6 energy damage for each +1 in your Inspire Courage.
- Martial Study from Tome of Battle can get you one of the excellent maneuvers from that book. What you’re looking for is a White Raven maneuver, which focus on being a commander: tactical strike is a single-attack standard action for +2d6 damage, and adjacent allies get to reposition themselves 5 feet, which is rather nice, for example. The reason you want a White Raven maneuver specifically is because it qualifies you for Song of the White Raven, a feat from the same book that allows you to start Inspire Courage as a swift action.
For more on getting more out of the bard class, consider this answer. Bard is a class with a lot of very good options spread out across a lot of books, so it’s helpful to know what’s out there.