If you don't have the time to shop or money is an issue, jeans, a long sleeve shirt, lace up shoes or boots and a straw cowboy hat will do fine. There isn't any club around here that wouldn't welcome you with that. The hat's not required, but it will save you from doing the "hot brass dance."
I do some shooting with suit pants and vests. They came from various Salvation Army shops, Goodwill, etc. I looked for 3 piece suits and gave them the suit coat back. Usually the cost is under $20. Not necessary, but the belt loops were removed and I put bachelor buttons on them so I could use suspenders. Bachelor buttons are easy to install, you hammer them on. Suspenders can be bought in the same stores for about $1 apiece. If you see a collared shirt you like, they are easy to convert to collarless by stripping the winged part of the collar off.
Occasionally, you can find hats and boots, but they are rare at the Salvation Army and Goodwill stores. However, they are occasionally available in thrift stores and antique shops. I think they drift into antiques shops with other stuff, and the owners want to get rid of them. I don't thing I paid more than $20.
I've more expensive stuff now because of gifts and shopping at bigger matches, but with the expense of starting with guns and leather, it was nice to be able to get into right look without too much money.