Brownells sells them. Brownells - World's Largest Supplier of Firearm Accessories, Gun Parts and Gunsmithing Tools
I will look tonight when I get home and get a part #
Find a buddy who has a reloader to drop a wad and some shot , but NOT the powder or primer. And somehow clearly mark the shells so you know they are inert.
I do/did the same thing with pistol rounds for practicing reloads (bullet only, no powder, no primer.) I'm very aware of weight differences and I want my training aids to mimic, as closely and safely as possible, the real McCoy.