I'm not sure on the best place to buy a clam but if you read the Clam FAQ at the top of this fourm it will give you good things to look for when buying a new clam
If your worried about keeping them, I usually recommend trying Durassa first - they need less light, and it tends to be easier to tell if their happy,.. but if you successfuly keep SPS then I would recommend just going with one you like the color of.
Durassa like to be placed in the sandbed,.. Crosea, and maxima like the rock - and need a bit more light as well. The biggest clam related problem I've had are fish that nip at them,.. make sure all your fish are clam safe first
Whiskey