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.
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!