Acid resistance can be obtained by wearing yellow dragon scales, yellow dragon scale mail, or an alchemy smock. It can also be obtained by polymorphing into any acidic monster, as they are the only monsters to have the intrinsic.
Sources of acid damage are as uncommon as acid resistance itself - even in scenarios where it can do significant damage, you can often easily avoid it by other methods (e.g., ranged attacks for acidic passives, and reflection for acid breath). Yellow dragon scales may still be worthwhile for pets and polyforms with multiple hit dice, particularly mind flayers.
The following information pertains to an upcoming version (3.7.0). If this version is now released, please verify that it is still accurate, then update the page to incorporate this information.Per this commit, monsters now gain acid resistance from a worn alchemy smock, allowing for more variety in equipping humanoid pets.
In SLASH'EM, the alchemy smock has been replaced by the lab coat, which also provides this extrinsic as well as MC3. Some new monsters deal massive amounts of acid damage, most notoriously the giant shoggoth, making this property more desirable for an ascension kit. Though still not a must-have, a player who has magic resistance, reflection, drain resistance, and magic cancellation through other means may opt for a lab coat or yellow dragon scale mail just to be on the safe side.
In FIQHack, extrinsic acid resistance protects your items from corrosion.
In EvilHack, due to the presence of monsters such as acid spheres and buffs to existing acidic monsters along with the addition of the highly-damaging acid blast monster spell, acid resistance is far more vital in comparison to vanilla. Yellow dragon scales and scale mail are thus far more valuable. Dragonbane is also changed to a pair of dragonhide gauntlets that provide acid resistance in addition to the standard reflection and warning of dragons, making it a highly sought-after artifact for most players.