You can try Craigslist for used tack/clothes. Depending on which Western discipline you're doing, you may even be able to shop at Goodwill. I do reining in boots, jeans, belt, western shirt, and hat. For my shirts I've got several that are J Crew, Banana Republic, etc button downs in fun prints and colors. They fit the bill and mean I don't end up wearing the same "in" shirts as everyone else.
Sho-n-Off. Elegant Show Styles. Showtime. Return to the Ring. Showcrazy. Heather's Hot Couture. Showfancy.
All are websites with consigned and new items. Some have tack as well. I bought a beautiful and simple shirt that zipped up the front from Sho-n-Off, added my own crystals, and get tons of compliments. I'm a size 10/12, and it cost $200. I'm 5'9" with gorilla arms, and it fit perfectly. Usually I have to take in the side seams, which can be a pain.
Showtime will often mark things down a lot, but you have to check often because their stuff will sell fast. They will let you return it if it doesn't fit, too, which lots of places will not do.
You can get Lisa Nelle items on ebay for less than their website, but you can't return the ones you buy on ebay, if I remember correctly. Their jackets are beautiful and well made.
One more thing. The trend now is going to simpler shirts and jackets, unless you're talking Congress or the World Show. Plain cotton shirts, impeccably fit, are everywhere in the show ring now.
I went to a show a couple of weeks ago and saw a woman with a cream oxford shirt on, and buckskin colored chaps, on a sorrel horse, with a hat that matched her chaps. She had a scarf at the neck, ends tucked, and looked fabulous. Classy. She stood out in an arena of bling.
Saddleup, the simple, classic look is one that I'm shooting for. When I make my show debut it will of course be with my helmet, and I think the look will be easier to carry off if I'm simply and tastefully dressed.
But I sure do love to window shop! Keep the awesome sites coming!
Oh, and I just thought I'd pass on that if you are looking for nice plain oxford type shirts, a good place to check is www.jiffyshirts.com - They have some limited colors on clearance right now for $3.80, and a large selection of colors at $16.38. I know the plain zip-front ones are more in style but for those prices you could really stock up. Add a nice scarf and the chaps and saddle blanket you already own and you can still be in the ring under $20!