I had the same problem with my photography prints and paintings that were odd sizes. Regular stores only carry basic frame sizes that rarely match what you actually need. I found
custom-sized frames online that let you input exact measurements and choose from different materials and colors. The process was really simple - you just measure your artwork and select the frame style you want. They cut everything to your specific dimensions and ship it ready to use. The quality was much better than store-bought frames and the price wasn't much higher