they will eat any meaty foods. you could get fish from the supermarket and feed them that. shrimp, scallop, squid, clam. btw, i believe puffers should be given snails from time to time so that they can wear their teeth down. i'm not sure if it's true about all of them though. i have heard of people having to remove the fish so that the teeth could be clipped down with some sort of coral cutters.
good luck with the undulate. i'm glad the two get along. those fish are amazing looking, but the ones i have seen have been insanely aggressive. they are, by far, the coolest of the triggers!