Believe me when I tell you that you probably don't want a Harpiosquilla. Yes, they are probably the third largest stomatopod, but they are not particularly hardy, live in mud burrows and don't do well without a burrow, are nocturnal, and a large one would not live too much longer.
A better species if you want they absolutely largest stomatopod is Lysiosquillina maculata. This species is found from Hawaii to East Africa, is common in very shallow water, and frequently shows up at importers. Ask your supplier for the yellow and black banded mantis shrimp. I've purchased several this year. They also simply set around burrows, but they seem happy in a 30", 3 inch diameter of PVC, love frozen shrimp and live for years (I have one that we have had in the lab for 6 years and it was probably at least 10-15 years old when we got it. This is the largest off all stomatopods - 42 cm is the record.
The largest species in the Gulf of Mexico is another Lysiosquillid, but it only gets to about 25 cm.
Roy