I never see them in stores here in NY, always have to search the internet. Petco has a few and they are always doing deals every other week discounting them.
I got my Crocea and Derasa from them. Both were nice sized 3-4 inches. My derasa died after i tried moving her to my new tank and the wife interfered and i dropped her back in the tank. the fall was a big shock and she didnt make the move. My Crocea is going strong though, has grown about a half inch and has been attached very firmly to the rock i first placed her on. I count a half inch from the white showing on the edge of her shell.
I see other places carrying 2" Maximas but they go for $100 or better. IMO thats a lot for a 2" clam.
I actually caught a sweet deal recently (I think). Picked up a near 6" Maxima for $100. Thing is HUGE and looking great. I think im going to keep that source a secret though. LOL. But if you shop around you will find them.
Look on Craigslist too.... dont just search clams though, throw in corals or reef... things show up.