Calbot can be learned in my School #25560, simply join the school equip the move set that comes with it and use it on NPCs or players until the bar fills up. Remember you need to kill your opponent for the progress to count so I recommend finding a friend or using it on easy NPCs.
Alternate grindy method:
The NPC that gives Calbot can spawn anywhere in the game I believe and can be identified by his nakedness and his black mask (I'd check google for a picture but you'll know it when you see it, all he does is slaps). To find him I recommend running around either bird caller outpost or outside the tower, do a full lap then kill yourself respawn and repeat, when I finally found the NPC it was outside the tower on top of the bridge and took about 7 hours of grinding lol. Once you've found him just block his slaps until the bar is full then kill him and you'll have unlocked the calbot slap!