Not sure if this will work for you but I have my local mechanic get my trailer tires for me. He gets them at more than reasonable prices and it usually includes mounting and balancing. Maybe talk to one and see if they can get you them?
I had the same problem when I replaced mine. When I did find the right size I didn't like the price. I put car tires on that were close to the right size. Havnt had any problems been running them for two years. They seem to be wearing better and were quite abit cheaper.