I went to walmart today and bought some light brown stretchy pants. They look almost exactly like breeches, and the price tag said $14.87, but when the cashier rang them up, they were on sale for $3!
Here's a picture of me wearing them:
As someone else already said, you can probably find a nice show jacket at goodwill. And depending on how 'horsey' your area is, you might even be able to get boots there. I got a really nice pair of cowboy boots at goodwill for $7.99 once, and I get all of my western shirts there, including an almost brand new 20x shirt for $3.99.
If you can't find boots at goodwill, you'll probably have to get them from somebody or order them online, but I'd try to avoid ordering online if at all possible, because it's hard to fit them. The same goes for a show shirt, but I'd feel better about ordering that online.