A friend gave me a 30K BTU vent free propane heater similar to this
http://www.tractorsupply.com/procom-reg-dual-fuel-vent-free-blue-flame-heater-30-000-btu-4501498. I converted it to natural gas and made brackets to hang it on the wall. It works alright but the fan does not kick on (I need to investigate) therefore the areas furthest from the heater do not get warm. Worst case I can put a box fan out there to circulate the air. My garage is a little larger than yours. Check craigslist, I have seen some really good heaters there.