If in Ontario, go to the OSASF (Ontario Single Action Shooting Federation) website. In the Classified section, there are a number of firearms for sale, which may be of interest. Otherwise, as already mentioned by others, some of this site's sponsors may be good sources. Ellwood Epps is a good place to start. Likewise: Marstar. Also: Prophet River. There are others, but at least these will give you a starting point. Inventory is pretty lean at all the stores right now, but you may have some chance of ordering in guns.
For further information on how to get started in Cowboy Action, there are plenty of highly experienced folks on this site, that can answer all your questions.
Al