As mentioned in this fine answer, immunity to ability drain is available from the prestige class bone knight at level 8 from its extraordinary ability exoskeleton of undeath (Five Nations 119) and from the prestige class pale master at level 10 from its extraordinary ability deathless mastery (Libris Mortis 49).
Likewise, as mentioned in this fine answer, perhaps the most readily available source of immunity to ability drain is the held wondrous item talisman of undying fortitude (Magic Item Compendium 188) (8,000 gp; 0 lbs.), it providing the bearer extremely temporary immunity a few times per day.
Also, while not technically granting immunity to ability drain, the slotless wondrous item gray portrait (Champions of Ruin 43) (major artifact; 10 lbs.) sees the portrait suffer the ability drain instead of the portrait's subject, but neither portrait nor owner suffers any mechanical ill effects from that ability drain. Whether such ersatz immunity is sufficient and whether the portrait can be discovered during an adventurer's career depend on the campaign. (Note that a DM may want to rename the gray portrait to the gray picture if he wants to hew more closely to its source material.)
Finally, the 4th-level cleric and 4th-level druid spell sheltered vitality [abjur] (Spell Compendium 188), in addition to other effects, grants immunity to ability drain. Many one-shot ways exist for any creature to use the spell sheltered vitality, from an attuned gem of sheltered vitality (Magic of Faerun 21) (1,600 gp; 0 lbs.) to a grim skull talisman of sheltered vitality (Frostburn 47) (2,800 gp; 0 lbs.) to a rare and ill-advised spellworm of sheltered vitality (Dragon #343 80-1) (1,400 gp; 0 lbs.).