I love that you and your son are enjoying the research together - my daughter is only 4 but she learning alot about environmental needs of the crabs.
I would like to second what Grammarcia listed for shell sources. I have made shell purchases on ebay for green turbos that my crabs swapped into but the ones I got from ElsShells are better. They were lighter and prettier varieties of turbos and they have already disappeared with the green turbos left in their place!
Shells are such a beautiful and diverse part of crabbing - I don't think I will ever find enough!
Have fun