If there's an app that tells you about the availability and prices of steamed crabs, I can't find it. Until then, how about posting your tips here. Email 'em to me if you want.
Here's Timothy B. Wheeler in today's paper about the crab supply (plentiful & price) for the July Fourth holiday weekend.
Find places to have steamed crabs here.
And, who wants to design the button for a steamed crab app?