You might be able to buy some lumber from a farmer who is tearing down his old barn. That lumber CAN be pricey, however, if the yuppies around you are trying to decorate. I think it also has to be soaked or treated if a large quantity is to be used to get rid of any insect presence. (Interior walls - like an 1880s gun room. ) Not to mention ... old barns being remodeled aren't exactly on every corner.
I was offered, very graciously, a bunch of old, weathered barn siding with which I was going to do the very same thing. Then someone mentioned being sure to well treat the stuff in case you were inviting critters you'd rather not introduce into the home. . .
After checking into it with a 'bug man', I came to the conclusion that it would be safer and probably cheaper to use new lumber and grey stain. Besides, if it's supposed to be a contemporary structure (new-built in the 19th cent. that is), it would not look like the interior of an old mineshaft.
Check out if there are any architectural salvage people in your area. They usually are a wealth of source for stuff.
Another idea would be places that deal in vintage hardware and the like, an example being:
www.eugeniaantiquehardware.com
Our new room will have a western theme as well, with this lamp as one of the decorations. I was kind of surprised when the wife didn't balk at me buying it.
Don't forget to keep it loaded for bear.