sometimes if I have a frame that's the wrong size/shape for my art I center the piece on a complementary background (usually just a sheet of Kraft paper, but you could also use patterned wrapping paper or poster board) that does fit the frame
if I have a bunch of small prints that complement each other I collage them in a single frame. photo corners can add a fun pop of color too and look even more intentional
alternatively, I've had some good luck finding frames on eBay, mercari, and fb marketplace