I like to buy my crab at the Alemany Farmer's Market on Saturdays; if you go very early, you'll catch the pickup truck driven by the two women from Half Moon Bay who took the crab out of the water that morning before driving over to the market. I also like to buy it at Sun Fat Seafood in the Mission as they have some of the lowest prices I've seen. And I know most people say Fisherman's Wharf is "too touristy," but I've bought the most delicious crab right off the boat there for a great price.
Even though the Dungeness crab season runs from November to June, the bulk of the local catch is landed from November to January, which means the lowest prices. After January, most of the crab sold in San Francisco comes from Alaska and costs more.
When buying live crab, follow these rules:
1. The livelier the better! A listless or dead crab often has a bitter aftertaste due to it's own digestive juices permeating the meat.
2. The crab should seem heavy for its size.
3.The shell should be hard - a spongy shell means the crab recently molted (shed its shell) and will have less meat than a crab with a hard shell.