They've been eating live blackworms daily for the past few months without any problems. We haven't had much success feeding them anything dry or frozen, however it seems like they'll also eat live baby brine shrimp, and they would probably like other kinds of live food (white worms, vinegar worms) as well.
Assuming you're in the Seattle area, you can try buying a starter colony of blackworms from Aquarium Zen or Ivy's Axolotls and then breed more at home (we followed this guide: https://www.youtube.com/watch?v=s7HQVvRn69o). There are also several online retailers, including Ivy's Axolotls, which sell live blackworms online.