Catch them yourself, there are several ways.
I prefer to use a Opera Crab Pot (like this http://www.trademe.co.nz/sports/fishing … 994517.htm ) , You need to add weight to the bottom and a rope for retrieval. I bait it with KY or old bait (fresh bait works best) then drop it off a wharf or breakwater. Check the pot every 10-15 minutes. Also on shallow beaches, I wade out then throw the pot. Again after 10-15 minutes I relocate the floating rope and check for crabs. There are different styles of traps and pots, same of them are better for catching larger crabs but I like smaller crabs for bait.
Another popular way to catch crabs is to wade the beach and simply grab or scoop them up in a hand net. I rarely use this method but it does work on calmer surf beaches. Walking around in bare feet is another good way to locate them.
The crabs are not always about, although I normally get good numbers around change of light. I also suspect tide plays a role but I have not figured anything out for certain.