Check out where they sell mobile homes. Quite a few times they take in older mobile homes on trade then strip them down to sell the frame, aluminum, etc. Many of these older ones that they strip will have wall mounted propane furnaces that would be ideal for a garage. I sold one awhile back for $50. It was 6 1/2' tall and about 18" wide and 1' deep. We ended up keeping the furnace out of our house for the garage when we had a new furnace installed, and I put that in my garage.