If you are familiar with eBay you'll know that's an excellent place to look for horse show clothing. I would type in "showmanship jacket" under "all catagories", and I expect you'll find hundreds to choose from. I bought a wonderful Hobby Horse jacket last year for only $45 and it was in brand new shape! Of course you have to know what size you need, and the extra costs such as shipping, etc. I'm assuming you aren't an eBayer or you probably wouldn't need to ask this question, so if necessary perhaps you can find a friend with expertice in eBay to help you get started.
There are also many equine sites that carry clothing. They have fixed prices so you won't be bidding the way you do on eBay. I have had alot of success with TackTrader.com, also if you go onto Equine.com they have a clothing listing. Then there are many Equine Clothing companies that sell used show clothing. They can be fairly pricey, but there are some really nice outfits out there. Try Showtime Show Clothes, or Karen's Kustoms, both those sites carry used, consignment clothing. Also, if you go to Hobby Horse's site, you can look at their "Broken Hearts" sale items, they often have fairly reasonable prices for brand new items.
Good luck. I'm sure you can find something decent in your price range!