If if I were in the OP’s position, I’d get clothes from the thrift store or used at a match.
For the basics, shirt and pants, I’ve had good luck at thrift stores. I’ve made slacks or jeans more “cowboy” looking by cutting off belt loops and adding suspender buttons. I also converted a free trench coat to a sort of duster/greatcoat by cutting off the belt, loops, and epaulets and dirtying it up a bit. I wear that in spring and fall.
Thrift store dress shirts or button up shirts have served me well (although I have some favorites from WW mercantile as well). I look for patterns I like, and usually hide the modern collar with a bandanna/wild rag that I found in WW mercantile’s sale bin.
Most of my hats have come from either thrift stores or eBay. I’ve tried to buy beaver hats but if I found a wool felt hat I liked for cheap, I wouldn’t turn my nose up at it. Boots came from a local western wear store, but I’ve seen decent boots at thrift stores too. Just make sure they’re comfortable to stand and walk in, because if they’re not, even free isn’t a good price.
The great thing is that most of my thrift store clothes can go right in the rotation with my purpose-made CAS clothes and still look good as long as I add a few accessories. (Wild rag, suspenders, etc)