Pendraken Miniatures has a Plains Wars line which will no doubt be of interest to 10mm Old West gamers.
Sparks: Sunburned and Rusty from Cumberland Games and Diversions. Sparks are paper miniatures rendered as a font so that you can print them out at any size.
Vulture Gulch Western Denizens from Microtactix are cardstock figures available for immediate download from RPG Now.
Thoroughbred Miniatures has a line of 10mm civilians to populate your N-scale cowtowns.
Peter Pig makes the original line of 15mm cowboys. Available expeditiously in the US from Brookhurst Hobbies.
Minifigs has a 15mm line called Best of the West with lots of indians and pret’ near everyone is available both mounted and dismounted.
Irregular Miniatures has a line of American Indians figures, which includes US Cavalry troopers and an odd cowboy or settler or two. Also check the American Civil War and Colonials lines for interesting characters.
Freikorp 15s, now owned by QRF Models, have the Yellow Ribbon Range which features indians, cavalry, settlers, buffalo, and a stagecoach to chase.
Jeff Valent Studios produces miniatures for Deadlands and the Great Rail Wars. The line contains typical Old West types and the more fanciful denizens of the Deadlands milieu. Look for some great steampunk vehicles, including an ornithopter, steam wagon, and auto-gyro.
Redoubt Enterprises specializes in British colonial figures, but also has a line of American Civil War figures which might be appropriate for Old West gaming. They also sell a 25mm train which is beautiful.