Heater for Small Greenhouse

[HDCochran]

My orchid collection has outgrown my orchidarium and I am building a small 12' x 12' greenhouse and will need about ~10,000 Btu/hr of heat to keep it warm enough next winter. I cannot find a ~10,000 Btu/hr vented propane heater. Does anyone know of a source? Alternatively, can anyone provide reliable information on any detrimental effects on orchids of using a vent-free propane heater? I've read arguments on both sides of this question.