I collect shrimp and crabs from sargassum seaweed at the beach and crabs, hermits and snails from the keys (when I go on collecting trips) for my mantis. I just throw everything in his tank and he kills and eats it as he wants to. When I see his supply is running low, I make another trip to get more!
I also suppliment Roy's (my mantis) diet with frozen foods I soak in selcon. He seems to love Flounder and Oysters that I prepare this way.