Most seafood shops will have blue crabs as soon as the local harvest gets strong. It may be a little early yet. If you'll be staying anywhere near Murrells Inlet, I recommend either Harrelson's or Murrells Inlet Seafood. In the past, at least one of them has also carried crawfish. I think it was MI Seafood, but not sure.
As far as seafood restaurants, almost all will offer crab cakes, and many of the casual ones will offer soft-shell peelers in season. Steamed crabs seem to be a little harder to find. Inlet Crab Shack in Murrells Inlet may have them. For crawfish, check out
Flow's (http://flosplace.com/site/ - broken link) in Murrells Inlet. They feature cajun style seafood and crawfish is one of their specialties.