Tacktrader.com has some great deals if you don't mind used stuff. Show jackets are soooo expensive so I think used is great. There's some people that sell the stuff they made on there and keep it pretty reasonable too so it's easy to find new stuff too on there.
Keep in mind, for a show shirt, the latest fad is the minimalist approach. Just a simple fitted oxford shirt. It's showing up at the quarter horse shows and a little at the paint shows. Express has some great shirts for that for about $50. So if you can find some chaps that fit awesome, it really does look incredibly classy.