Matthew: If you are portraying a late 19th century cowboy who can afford them (Irish Dave is a Pinkerton man and they always seem to be flush with money) Cabella's bags are reasonable for that late 19th century appearance. If you are but a poor cowboy, or doing an 1870's impression you may wish to consider a "wallet". I have two references of cowboys from that earlier era using such and would be happy to provide them for you, and the image below.
![](http://i182.photobucket.com/albums/x80/Jhunt67/PersonaStuff006-1.jpg)
Although some may refer to it as a war bag, I don't see that nomenclature used much in the period and always thought of it as something like a sack that you'd tie off on the saddle horn.
Unlike a sack a wallet can be made up of any size out of about any material used during the period (a coarse cotton, linen, canvas) and can be thrown over the shoulder conveniently and balanced well for carrying much like a saddle bag - all for the fraction of the cost of saddle bags.
If you just can't stand plain:
![](http://i182.photobucket.com/albums/x80/Jhunt67/apachesaddlebags.jpg)
I have used my wallet at shoots for the last three years and it has proven more than adequate for the task. It is shown below:
![](http://i182.photobucket.com/albums/x80/Jhunt67/SaddleWallet.jpg)
I actually use it on the horse also. A fine historically correct solution and inexpensive.