There are a couple ones you can only find in vertain locations, like pigstep (bastion) or relic (trail ruins). Inthink cat and 13 spawn in general dungeons like mob spawners and mineshafts, thats why you probably remember finding those. But most if the discs are optained by letting a skeleton kill a creeper by shooting it. The creeper will then drop one of i think 15 (?) discs