“How to heat crawl space? – recommendation needed”

Photo by PoPville flickr user Mr.TinDC

“Dear PoPville,

I recently had the 3-foot high crawl space of our row home/3-unit condo building conditioned, with spray foam used on all perimeter walls, and then sealed the crawl space entrance as well as any vents, and added vapor barrier to floor. It was recommended that I add a heat source to the crawl space so that my floors are not freezing cold (I am in first floor unit above crawl space). Can anyone recommend a person to evaluation my situation and make a recommendation on how to safely provide heat to the crawl space?”