There is a section on www.mrheater.com that sells a variety of hose attachments. Did you check with Mr. Heater's customer service to see what they recommend?
However, from what I know about unvented tank top radiant heaters, they are not recommended for indoor use due to the risk of Carbon...
Mr. Heater is a Cleveland, Ohio based company that has been manufacturing overhead tube heaters for years. They have a call-in service help department too. You can buy their garage heaters from Northern Tool and www.heatmygarage.com. You can also get more info from www.mrheater.com
I found a Website called heatmygarage.com. It looks like it has a lot of information about heating garages, definitions of radiant vs convection heat, tube heaters, heater specs, etc.
You can order heaters from this site as well.