Thanks for posting on the forum...
There are oyster farms on all three islands and you'll have no problem finding fresh Dungeness crab no matter which island you go to. Of course my favorite way is to drop the crab pot but if you don't have a license, a boat, and a pot you can always buy them from the fresh seafood departments at the local grocery stores.
San Juan Island:
On San Juan Island the only oyster farm is about 10 miles from town near Roche Harbor. As of June 16, 2009 they appear to be open Friday through Sunday. Visit their website and call them for exact hours here: http://www.westcottbay.com
Raw and barbecued oysters from Wescott Bay are also served fresh from the following locations in downtown Friday Harbor ( the ferry lands in Friday Harbor )
Downriggers Seafood Restaurant: located on front Street half a block away from the ferry landing.
Friday's CrabHouse: located just opposite of the ferry landing in downtown Friday Harbor.
Steps Wine Bar and café: also located in downtown Friday Harbor.All Friday Harbor Restaurants.
On orcas Island:
About 17 miles from the ferry landing is the Buck Bay Shellfish Farm ( no website ). (360) 376-5280
Article and photos can be found here: http://mixedgreensblog.com/2008/10/20/on-the-local-table/fresh-shellfish-and-yoga/All Restaurants on Orcas Island
On Lopez Island:
The Jones Family Farm http://www.jffarms.com/
produce oysters and shellfish from their farm but I'm not sure if they have a retail shop. They are located on 1454 Mud Bay Road, Lopez Island.All Restaurants on Lopez Island