The DM will likely rule most instantaneous buffs are accidentally instantaneous; nonetheless, a few withstand scrutiny
As this fine answer mentions, the problem with buff spells that have durations of instantaneous is that such spells—when they exist—are going to be totally unbalanced, extraordinarily expensive, or wholesale setting-changers, and—when discovered—will likely be banned or nerfed by the DM.
However, if you want, below are the two spells that can be considered buffs that I immediately thought of. Both spells require the reader to approach them from an extremely strict perspective, and if accusations of being a rules lawyer hurt your feelings, you shouldn't even try to convince your DM to allow them into the campaign as they can be read.
The 4th-level Clr spell mystic aegis [abjur] (Player's Handbook II 120) has the entries Casting Time: 1 immediate action and Duration: Instantaneous. Its entire description says, "You cast mystic aegis immediately when you are targeted by a hostile spell. You gain spell resistance equal to 12 + your caster level against that spell." A generous reading sees the aegis spell's caster forevermore gain SR against that spell and any future castings of that spell—making it useful for casting between adventures to gain SR against specific spells the PCs know their enemies will be using, for example,—, but I suspect most DMs will read that description as granting the caster SR against that casting of that spell. Ask the DM.
The 6th-level Sor/Wiz spell opalescent glare [evoc] (Spell Compendium 150) has the entry Duration: Instantaneous. It's description leads with the following sentence: "Inspired by the deadly gaze of the noble ghaele eladrin, you gain a gaze attack usable against creatures within 60 feet." I suspect many DMs will likely rule (as this DM has) that the spell allows the subject to immediately make this gaze attack, and the gaze attack's effect is instantaneous. However, that isn't what the spell says.
(You can compare the glare spell with the 2nd-level Drd spell blinding spittle [trans] (SpC 32) that says, "You spit caustic saliva into your target’s eyes with a successful ranged touch attack," and the more similar 8th-level Sor/Wiz spell bodak's glare [necro] (SpC 34) that says, "Upon completion of the spell, you target a creature within range that can see you," both of which also have instantaneous durations.)
Other than these, there's, like, the 9th-level Sor/Wiz spell kissed by the ages [necro] (Dragon #354 54) that stops a creature's aging—a buff of sorts—, and the 9th-level Sor/Wiz spell wish [univ] (PH 302-3) that, in addition to its listed effects, can pretty much do anything the caster wants… if you don't mind the DM "pervert[ing] your intent into a literal but undesirable fulfillment or only a partial fulfillment."
There are spells that create things, of course. For example, the 2nd-level spell fire shuriken [evoc] (SpC 92) has the entry Duration: Instantaneous, and the effect doesn't say the spell's magical weapons ever disappear, although they disappear when they strike a target. While a strict reading suggests that they can be thrown only by their creator, I guess an assassin could create a bunch of fire shuriken effects during his downtime. However, free weapons aren't much of a buff, really.
And there are spells that change things forevermore—like the 4th-level Drd spell awaken [trans] (PH 202) and the 3rd-level Clr spell planar familiar [trans] (Spellbook Web column "Planar Familiar")—, but those aren't particularly ripe for use on PCs (although with some work I suspect someone can probably force them to be).
In sum, increasing caster level is a better route than trying to leverage an instantaneous effect into a long-term buff or researching an original spell that grants a instantaneous buff. If angling for long-term buff spells, a really high caster level will, essentially, render moot most attempts at dispelling your buffs, and having a really high caster level won't in and of itself ruin the setting's verisimilitude, although making your spells impossible to dispel—and from here I speak from experience—may still irritate the DM.