I've never seen them for salebut they're easy to catch if you have a rake. Just walk along the shore, rake a bunch of sand at the shore line, let the water rinse the sand out. More often then not you'll end up with some sand fleas in the rake. If you hit a spot you should have enough for a fishing trip in a half hour or so... they can be a little tricky to keep alive. Use damp sand only and do not cover the little buggers with water.
The rakes are made specifically for the purpose... I bought one for around stainless one for around $35. What I don't know is if they're around this time of year.. they are abundent almost everywhere in the summer.