Known also as Manila Clams, almost all Oriental Supermarket has them. Just saw plenty at T&T this evening. The plate of Zuppa Di Paesce (Italian Seafood Soup) I just had at Shiso Leaf Cafe at J-Town also used plenty of them!
OP posted a link. Species: C. antiquata. This is neither the Manila (asari) nor littleneck clam. From my experience, there's not a huge difference between the two, but I've never had this specific clam (though it seems similar).