Welcome!! It sounds like you're on the right track for having a nice setup. My recommendations:
Ditch the commercial food completely. They're often full of nasty preservatives and unsafe ingredients. You can feed fresh exclusively, and they can eat pretty much anything we eat, minus seasonings/spices. Under our food/water section (
http://hermitcrabassociation.com/phpBB/ ... m.php?f=25) there are sticky posts on safe and unsafe foods, as well as a general guide to nutrition. You can also feed dried food for convenience, and there are several stores that exclusively cater to hermit crabs. Check out the shopping resources at
http://www.hermitcrabassociation.com/pa ... pping.html
As far as shells, they're often times difficult to find in stores if you don't live near the shore. A lot of us buy them online. About how big are your crabs? Since they're purple pinchers, they'll probably prefer shells with round openings; specifically turbos. Again, on the shopping resources page, there are several sites that sell shells. The Hermit Crab Patch has a good selection of shells and food.
Feel free to ask any questions you may have
anyway back to the more happy stuff... we just got 3 Hermit crabs 2 jumbo and 1 medium. Their names are mr crabs ergo and Jurassic. They are purple pinchers. They are not shy at all come right out! 
They are currently living in a 20 gal. Their substate is sand and Eco earth plus some frog moss in the corner. It's about 6in deep (but it does slope down to create a hill type thIng) They have a salt water dish and a fresh water dish. I'm feeding fresh foods once a day at night and commercial foods every other day. They have i cork round to climb on. They also have a aquarium rock. They have some fake reptile vines too. It's very hard to find extra shells in our area. We found only three so far and that was after fiecly hunting through 3 pet stores 
we have a low watt CHE to keep the temp up. We have plastic wrap over half of their cage to keep the humidity in. We also mist them three times a day with dechorinated water (just like in their water dishes). We also have a sponge in the water dish (doesn't take up too much room they can still bath) and we sanatize it often. Feel free to nicly critique my care. 
