Woha,
What an incredibly difficult question to answer. Actual "Cowboys" were a very small percentage of the population. Most true cowboys didn't have a "wardrobe" to speak of. Maybe two pair of socks (if any), two pair of drawers, one shirt, a vest and a coat. Maybe a pistol and gunbelt tucked away in saddle bags with everything he had except what he was wearing, stuffed in the back of chuck waggon.
The same applied to anyone who primarily lived on a horse. Outlaws, traveling lawmen, Tinkers etc. Townfolk were a mite different. Basically the same clothes, just clean. Collar shirts go back as far as .... well a long time back. Collars were common well before the 1880 while "collarless" were also normal. Shirt (oops) pockets were also seen way way back.
Don't forget, the 1873 Colt was NOT everyones firearm nor was the the 1873 Winchester, the gun that won the west. The shotgun was. Aw heck, I could just go on forever so I also suggest you snag a copy of the book recommended by River City John. Packing Iron is also a "must read."
Coffinmaker