Replace reusable air filter with disposable, when no size matches?
You can cut the electrostatic filters (like Filtrete) down to size without altering their effectiveness. Just buy a size that is larger and cut it down with tin snips and/or scissors. When I do it (and I have done it many times) I use painter's masking tape to "seal" the cut edge.

If you install it in the plenum of the forced-air unit, make sure you "seal" the cut edge well so the blower does not blast fibers or pieces of the filter media out into your ducting. If it installs in the return-air grill it may not be as important because the force is usually less.

Home Depot and Lowe's only carry sizes that sell well, check 3M's web site and see if they make them your size. Then go to your local mom-and-pop hardware store and ask them to order a few. Try to shop at the local shops instead of the "big box" when you can.