i would check withyour propane dealer and see if they have something.....i personally use an unvented procom blue flame heater and have had no problems with it....
Reznor makes a good heater... Probably not cheap, but you don't want to skimp on heaters.
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I have had Modine heaters most of my greenhouse life, but went with a separated combustion unit from Reznor a few years ago. It's a V3 UDAS model - basically the same design as the Modine Hot Dawg, but with the separated combustion.
When I replaced an equivalent BTU Modine with it, my propane consumption went down 40%.