I have seen them, but only by error, and the shop keeper didn't seem to know what he had. So it's not unheard of. But as far as there being a NA captive stock of true bream (that is, not North American sunfish) I can't really say.
Being native to Europe, I wouldn't think they'd be too hard to import unless they're restricted, but i can't imagine there are many people even in Europe that keep them, for sale or not.
Josh