My usual seafood response: Ippolitos at 13th and Dickenson, may need to call a day (sometimes half a day) ahead. So not the burbs, but if you see the Samuels and Sons truck delivering to any store, they can get them for you too!
Fresh Market sells them. Since they are almost certainly canned (or jarred) no matter where you buy them you might want to buy the can yourself and make a fresh garlic sauce. You can also buy the empty shells if presentation is important.