Liechtam

For those who never found the answer or got linked to a guide that wasn't accurate and are STILL Looking for Drain Punch, here you go: Been playing through Johto, as it was just added somewhat recently, and I've found Drain Punch in two places apart from the GTL, you can pick it up next to this tower directly south of Moomoo Farm on Route 39. (In front of where my character is facing.) And you can also buy it in Blackthorn City from the Pokemart for ~2k cheaper than it's listed on the GTL.