You have a few choices
TMs pop up in the world of Pokemon Scarlet & Violet like daisies, and sometimes you’re going to just come across a bunch of very important endgame abilities. If you’re looking for Drain Punch (TM 073), you have a few choices.
You can get Drain Punch west of Levincia, on a small island
The first, and likely easiest option, is directly west of Levincia: the electric city with Iono as the gym leader.
To find Drain Punch, just leap over to the little island that’s disconnected from the main body of land, and grab the item capsule on the east side of the island. Given how early you’ll be approaching this zone anyway, you could just naturally come across it.
There’s a second Drain Punch TM capsule at the South Province (Area Five) Watchtower
If you need another one without fussing with the crafting requirements: or you’d rather go this route, you can find Drain Punch at the Southern Province Area Five Watchtower. Just climb up one level, and you’ll find it on the first ring before you reach the top.
Exploring this Watchtower will also net you a free fast travel checkpoint, so it might be a spot you want to visit regardless.