Costco used to keep huge plastic tubs of excellent chopped clams packed in clam juice (not sure I'd call it clam "stock," though: clams don't quite need to be cooked down for hours or days to get a flavorful liquid). Not sure if they still do.
Best/easiest way to get "fresh" is to steam a bunch of clams in a pot. Eat clams, reserve liquid for your class. Again, Costco (on weekends) typically sells large bags of very fresh New England cherrystones, mahoganies, or whatnot dirt cheap. Any seafood store should have fresh calms, though. Check Chinatown or Argyle.
Don't forget to clean them. You will want to strain the stock to get rid of any grit.