Well, there are some sites like subguns.com that specialize in full auto type guns- and they have classified ads to sell stuff.
The two LARGEST on-line gun acution sites are gunbroker.com, and auctionarms.com They permit you to register, put together an ad, insert photos, and sell stuff in general. They get a small percentage of the sale.
The other way would be to search for some of the WWII re-enactor groups. It gets REAL hard to find some things that they have to round out their equipment.