Don:

Thanks for your question. I've owned a 4-bore double shotgun in the past and just closed the deal on a 4-bore double rifle by R. Hughes. I have about 35-40 rounds of new-production brass that I bought from Rocky Mountain Cartridge (they are on the web). I believe the cost is now about $25 per round. I also have some vintage cartridges in my small collection. I  bought an original Greener for nearly $500. It is not for sale, but I wanted you to get an idea of prices of the old stuff. Just for your shelf, calling RMC is your best bet.

Good shooting,

Cal