Here in Denver, we have a thrift store for building supplies, Bud’s Warehouse. I don’t know if there is anything like it anywhere else, though. Just like any thrift store, it’s hit or miss as to what you can find, but it can be worth while. It’s mainly leftovers from other building projects, or things that are still in good condition from remodels, etc.
You might want to see if there is anything like it near you. We needed an extra tall screen door to fit a doorway with a step, & they had one at Buds!