First off welcome to the forum. You should take a look at your local adds in the news paper, always adds in there. Also, online such as horsetopia, or dreamhorse among a few sites, have specific sections where you can search for local trailer adds.
Make sure you take into consideration the size of your horse when looking for a trailer. The last thing you want to do is to buy one that is to small like a lot of the cheaper trailers out there. They are quite petite in size.