Go to a food store which is well-stocked with seafood, or a seafood store, and get a bag of live black mussels. They will be on ice. Take them home and freeze them. Take a frozen mussel, thaw it in aquarium water, split it open, toss it in the tank and watch your puffer get big and fat.
I feed one a day to my display. Fish tear it to shreds, and it saves me a small fortune on frozen fish food. This is actually better food than 99% of what you buy at the LFS. You will probably only need 1/2 mussel a day - for now.
Just see how much they love this!
You can do the same with canned oysters. But, unlike the mussels, fish must learn to like them. You must rinse the oysters under a tap, place them in a thin sheet in food bags and freeze them. The thin sheets are easier to feed than a chunk of oysters.