We have an odd ball size also. When I finally found my size, I bought a whole case. I really prefer those pleated and treated filters.
Until you find one that is the exact size, you can do the following ONLY if it is one made with reinforced wire structure. With sissors, you can cut down a too large size to the size of yours. It is imperative that you cut the filter that exact size. you do not want dirt and lint to by pass the filter and go into the system.