Stomatopod.com is one of the best sites I can think of, though honestly they don't have much in stock ATM (They have quite a few orders due soon)
mantis shrimp should be fed a diet of 2-3 frozen marine foods, soaked in selcon once a week or so. Feed once every 2-3 days. Feed live foods at least once a month for stimulation, though more is better.
Smashers have a calcified club on their raptorial appendage to "smash things" and spearers do not have the calcified club but have much longer and barbed raptorial appendages so they can slice and stab things. If you go with a spearer you'll want P. ciliata as they are about the only active and easy to care for spearers available.
Dan