Mantle of Rage
This random PDF file on Wizards’ website includes the mantle of rage offers a no-frills, 1/day rage as a 1st-level barbarian for just 6,000 gp. Easy, excellent approach.
(Thanks to @HeyICanChan for pointing out this PDF and mantle of rage.)
Mask of Fury
Dragon vol. 324 has the mask of fury, a 12,000-gp helmet that grants
the ability to rage, as the barbarian ability, twice per day.
It requires a standard action to activate unless you’ve been wounded in the last hour, but unlike the following options, provides actually-barbarian-rage and does not require particular alignment.
(thanks to @Forrestfire for pointing this out.)
Wolverine’s Rage
This [wild] feat from Complete Divine expends a use of Wild Shape to give you a Rage-like ability. The specific benefits are +2 Str, +2 Con, and −2 AC, and it lasts 5 rounds. You can only use it if you’ve been damaged in the last round.
It’s not actually called Rage, so it would not qualify for anything that requires Rage. Still, a decent answer.
Gutworm (Fiend Folio)
The Gutworm fiendish symbiont (Fiend Folio pg. 220) grants you Rage, a +2 inherent bonus to Constitution, neutralize poison 1/day, and makes you, well, fiendishly hungry (with associated penalties). If you’re not Evil, the Gutworm (or any other Fiendish symbiont) can also drive you mad. Basically, this is a bad idea unless you’re Evil, since you’ll wind up in Personality Conflicts with the thing otherwise and that leaves you open to it controlling you instead of the other way around.
If you are Evil, though, this is probably the best choice, since the bonuses are much better than Wolverine’s Rage and it’s real Rage for prerequisites and the like.
The trick with that is you’ve got to find one. A casting of lesser planar binding should get you one, and convincing it to do what it is naturally inclined to do shouldn’t be hard (actually, arguably you could use LPB to bring it to the Material, then just let it get free and attack you). Of course, lesser planar binding is a 5th-level spell, which is extremely difficult to get in E6 (and a DM really probably should ban any methods of getting one that do work).
Without LPB, it’s an Outsider with relatively low HD, so you should be able to get relevant information for your campaign with a fairly-easy Knowledge (the planes) check.
Magic of Eberron suggests that symbionts get treated as if they were items for the purposes of Wealth By Level, which is reasonable, but it only covers the Daelkyr symbionts from Eberron books, not the earlier Fiendish and Undead symbionts from Fiend Folio. This poster suggested 10,000 gp as a reasonable value for the Gutworm, but that’s purely homebrew.
Bloodrage Gauntlets (Magic Item Compendium)
If that’s not your style (and let’s face it, it’s got a lot to recommend against it), Bloodrage Gauntlets from Magic Item Compendium can grant you an ability similar to Rage, but it also damages you over time; they’re pretty cheap anyway.
Wildling Clasps (Masters of the Wild or Magic of Faerûn) can allow you to keep a body or neck-slot item while in an alternate shape, which means you’d need a custom “Bloodrage Amulet” or something. The magic item guidelines suggest that this should cost 50% more than usual, but for something like this I think a reasonable DM will waive that.
Barring that, you could pay 100% more (i.e. double) for it to be slot-less, like an Ioun Stone or something. This will (probably) negate the need for Wildling Clasps to begin with. Might be cheaper anyway.
Rage (Player’s Handbook) and the Wand-wielding Raven Familiar
The spell rage basically mimics the Barbarian’s Rage, explicitly referencing that class feature. Unfortunately, it is a Bard 2, Sor/Wiz 3 spell, takes a Standard Action to cast, and only lasts for as long as the caster concentrates, plus 1 round. This is not really a great tactic. You might be able to get a kind of intelligent item that can cast this spell and concentrate on it for you, but that would entirely be up to your DM.
Better might be to take Obtain Familiar (Complete Arcane) as a feat, get a Raven (which can talk) for 100 gp, train yourself (and thus the familiar) in Use Magic Device (Apprentice [Spellcaster] from Dungeon Master’s Guide II can get it as a class skill), and give it a Wand of Rage (Bard version) for 6000 gp. Every combat, have it spend its first turn casting rage on you, and continue concentrating on the spell for the rest of combat.
You basically burn 120 gp per combat, though. This isn’t awful, but it isn’t great, either.
Shifter (Eberron Campaign Setting)
The last thing I’ve got is the Shifter from Eberron Campaign Setting. Their Shifting feature is sort of half-shapeshift, half-rage. Sadly, they’re really not that good a race, though, and don’t really grant what you want.
Unfortunately, those are the only things I can find that do not involve class levels in something that isn’t Druid.
