I have 3 Geothermal units with odd filter sizes. I found what i needed here. The quality is quite good and less expensive (~1/2 the price) than the HVAC company charges.
While the above dimensions are "nominal" dimensions for my geothermal system, they are actually supposed to be just slightly (~1/8") smaller in all directions. The FilterBuy air filters have to be a little smashed in there to fit, but they work perfectly and keep the heat-exchanger and the rest of the inside of the system clean and shiny. Considering what that system originally cost, I'm very pleased to see that these filters do exactly what they are supposed to do.