I've been picking them up at the local gun shows for the last 25 years or so. Also tend to find them in the curio cases of off-the-beaten track antique shops. Scored a whole cigar box full of late 19th- / early 20th-century stuff in Canon City like that for a song in 2007.
I got my .700 Nitro ($50) and a cool .577-450 Martini paper-patch ($30), both Kynoch, at Cherry Creek Shooting Ctr when Alan Duckworth was still running the place, probably 1999 or so.
Since you're local: there is a Rocky Mountain Cartridge Collectors' Association. They have one show/meet per year in July in Golden. The president of RMCCA is in Englewood. He does not do email or internet, but is always buying, selling, trading. If you'd like his contact info, PM me.