I'm not sure, but I think Dangerous Dave (no relation, I promise) The Old Western Scrounger sells some black powder 12 ga rounds. I've dealt with him on the phone, he's a nice guy and very knowledgeable - RCBS's customer service and Ballistic Products sent me to him when I had questions about reloading brass shotgun shells.
I don't have his website handy, but it's not hard to find - search for "Old Western Scrounger".
You also might want to try Ballard Cartridge Co. (don't have their site handy, either).
I know I've seen them somewhere, recently...