Have been getting my fill from Fresh Fields, they usually carry them in winter. Limited variety though they only carry Malpeques from canada and also one's from Chesapeake. Priced at $0.89 and $0.49 each.
Heck of a lot cheaper than eating at a restaurant and very fresh too. Only downside is you have to learn to shuck them, not difficult once you get the hang of it and make sure you use a oyster knife. I have a cemetery of good knives to prove what happens when you don't use one.