They are available on the used market. The guns are more readilly found in the UK, as it is still legal to hunt with them there. This English web site has a couple for sale, one double and one single shot (use the find function on your browser and look for "8 bore"). Get out your piggy bank, they don't come cheap.
http://www.micksguns.com/antique%20guns.htm
I understand that ammo can be built from modified Remington industrial shells. It should be possible to find a less expensive muzzle loader, if you are so inclined.
Sharptail