I don't remember the name of the company, but I believe that Shotgun news occasionally carries an add for black powder shotgun shells. It seems to me they were an English brand and quite expensive, but I think they may have had 20 guage. You might want to consider loading them yourself however. I have been loading them for a couple of years now and it's not too difficult and a heck of a lot cheaper than buying them.
"A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed."-Second Amendment to the Constitution