There is no Belly Drum TM in any of the mainline games. Rather, Azumarill learns the move Belly Drum as an egg move, either being passed down from breeding or having it hold a Mirror Herb in a picnic with an empty move slot, where it will learn it if another Pokémon in your picnic knows the move.
Azumarill's compatible parents for Belly Drum can be found on the site here.
