How do you get abomasnow?

GBA Pokémon Games

Not available.

All NDS Pokémon Games

Trade, hatch from egg and level up to 40.

Diamond/Pearl/Platinum

You can catch it on the Mt. Coronet Peak but it is rare, so the easiest way is to catch a Snover, which can be found in route 216, 217, Acuity Lake and also Mt. Coronet Peak then level it up to level 40, when evolves into Abomasnow.

HeartGold/SoulSilver

From Pokéwalker.

Black/White(1&2)

Transfer from the above games.