I've been obsessing about this with my buffer bard, too.
For your money, the single best item is probably the ring of enduring arcana (C.Mage). It increases your caster level by +4 for the purposes of resisting dispels. It costs 6000g, which is manageable at fairly low level, and it's a flat, constant bonus ... always with you.
The ring of counterspells is good - it guarantees protection against the first dispel to come your way. Note that if you're in a psionics+magic campaign, it's no good against dispel psionics.
Rings of greater counterspells and spell-battle are also good - they offers ways to counterspell or block a spell as it's being cast, so you could just negate a dispel before it even hits you. The ring of spell-battle is particularly nice ... it lets you redirect a spell, so the dispel pointed at you can be retargeted on an enemy.
Anything that ups your caster level, obviously. An ioun stone or whatever. Clerics have a bunch of good options: prayer beads and the ankh of ascension come to mind. They're limited use, but good for making your critical spells really rock solid (and the prayer bead of karma is great for a morning buffing routine).
In a game with psionics, there's the dispelling buffer power, if you can swing it in an item (or an allied psion). +5 to the DC to dispel you.
There's also a metamagic feat ... Fortify Spell, in Complete Arcane. It works like Heighten Spell, except that every +1 to the spell level translates into a +2 to CL. Might not be worth it, but it's an option.
There's also Earth Spell, from Races of Stone, but it's probably not so good for this purpose.