The shell sets from THCP are a good start, and they have shells with both D shaped and round openings. Your crabs are both Purple Pinchers, so they'll probably prefer round opening shells like turbos. I think the favorites are striped turbos and tapestry turbos. Smaller crabs seem to like to wear green turbos..or at least mine do! I'm a terrible judge at shell sizing, so I may not be much help.

Naples' Seashell Company has pretty good instructions on measuring:
http://naplesseashellcompany.com/sizematters.html They're also a good place to order from.

You'll want to get a decent variety of sizes - I have teeny crabs in comparatively huge shells, and larger crabs crammed into small ones so you just never know what they will want to wear.
So...after all that rambling, the packs at THCP may be a good starting point for a good variety to see what your crabs prefer - once you find out, then you could order specific shell types/sizes.