Some of the best bases I have found are from garage sales. Artwork in frames where the frame is the goal, or loose frames, old plaques and trophie bases, serving trays made of every kind of material, old small furniture or objects that were used as decorative items on shelves etc.
When you go to a garage sale or thrift store with an eye on looking for odds and ends that would make a great base, things really start to open up.
Also the As Is section at Ikea if one is near you is ideal. They always have loose shelves, and parts of furniture as well as other items for cheap. I have even used old floor tile as a base once.