I have my seeds on a heating pad. The only other option would be my water heater -- it's old and so the top is warm, but I'm not sure it's warm enough.
Sorry Liz, I don't have any experience with P. japonica. My P. obovata is in a raised, east facing bed, which tends to be on the dry side because the nearby spruce trees provide partial shade and suck a lot of water out of the soil. It doesn't seem to be bothered by this, but is very slow growing, which I have been told is quite normal. It took about 4 years for it to produce a flower (this spring). I mulch with compost in the fall.