Check Kittery Trading Post's Used Guns section http://www.ktpguns.com/just select Muzzleload: Long Gun under action and search. Also, http://www.williamsgunsight.com/gunsights/813usedblkpowderguns.htm occasionally lists some nicer rifles.

http://cherrys.com/ped_rif1.htm

A lot of guns at Track of the Wolf are consignments and some are used - heckuva selection.

And there is always GunsAmerica, GunBroker, and AuctionArms

Sometimes some are listed in the classifieds of forums like this one - worth a check.