I reccomend Ebay! you can buy lots of nice stuff. You should always check the seller's feedback though. Most reputable sellers offer tons of pictures, and a page or two of information. (Saddle size, tree size, make, model, reason for selling, etc. etc.) I almost bought a Western show saddle for my Morgan this past spring, since I show western.. but I probably wouldn't use it more than a season, so it wasn't worth it. (I just borrowed one) I did buy a couple western bits though!
The thing about ordering saddles (especially western, where the seat can vary much more than eng) is you can't try them before you buy them.. So you don't know how comfortable you'll be in them. My advice is try a few friends' saddles for size and comfort and try to order one that is closest to those. I like Circle Y saddles, personally.