thawed out frozen krill works, as well as buying fresh shrimp from the market and cutting it up finely. There are a lot of good recipes from members too. Here's a thread with some: http://reefs.org/phpBB2/viewtopic.php?t=32050
I replied in the GRD thread, but just to put it here too, make sure you give them some really hard crunchy stuff every now and then. Puffers' teeth will keep growing constantly, and they need to be ground down. My last puffer would grind his away on some porous tufa rock I had in the tank. And he LOVED an old pipe-organ skeleton. hehe