As Benta said, almost always custom. Transformer design varies a lot between power supplies. Depends on the topology used, controller, input/output voltage and current, physical form-factor, etc. There many be some off-the-shelf offerings but they probably won't be just right.
Making your own transformer actually isn't all that hard. Just need magnet wire, maybe some triple-insulated wire (TIW), 3M 1350 polyester tape, and core/bobbin set.