I second facebook tack trader pages. You can get some pretty good deals on used breeches from folks that have outgrown them or changed disciplines, etc. You can even get some good prices on new ones that people bought in the wrong size. Just make sure that you pay through paypal for your own protection
Also check the horse forum classified sections. If you post your size and what you're looking for- "Size 26 tan breeches under $50" then you may find that forum members are looking to get rid of some.