I have a Dog Face Puffer that is extremely picky about what he will eat. Before buying it - the LFS showed that he ate frozen krill. Well, that is all that it will eat. Went to the ocean beach this summer and dug up some local crabs. They are called mole crabs and live in the sand at the surfline. They are sold on this site by the name of sand sifter crabs. I brought them (at least 50) back in a bucket of water and sand - I fed the last two to it this week. It is the only other thing that it will eat.
The mole crabs go by several local names. Sand crabs, Sand fleas (bait store name), Mole crabs. The Puffer REALLY ate them up. Not even a piece of shell would go uneaten. Was quite funny to see him chasing the Niger Trigger around and taking the crab out of the Trigger's grasp.
One thing - I had to crush the crab before putting the first few into the tank. The Puffer was a little slow at first to fugure out that it was food. But after the first week, it would try and jump out of the tank to get to the crab in my hand.
If you are in Puerto Rico - a trip to the beach with a bucket and a shovel might get you some results. The crabs have a tough shell to help with the teeth. I know that the crabs are common from along the East coast from NJ to FL.
Good Luck