Hot Springs around Bulgaria?

I'll be travelling through south-eastern Europe (former Yugoslavia, Albania, Bulgaria, Romania) in 2 months on motorbike. I've heard there are hot springs / spas in Bulgaria, and it might be nice to visit (especially after a few days in the saddle). Natural hot springs would be nice (I can get a sauna anywhere), and open air, big pools would be cool. It doesn't even have to be Bulgaria, we're travelling around.

Where can I go to get this sort of thing?

Update: Things that would be cool: Outdoor communal swimming pool type hot spring. Natural (not artificial) heated water.Historic setting / 'something with some history to it'. Price is not too much of an issue, but gold plated towel rungs would be OTT :P Places in nice towns/cities would be nice, rather than somewhere in the middle of no-where. It would be nice to ride into a town, stay in a decent hotel, go for a decent sauna, go for a nice meal in a restaurant, etc. would be nicer than some excellent hot spring in the middle of nowhere. We'll be there in the first 2 weeks of September this year.

Recommendations of specific places (not just towns) would be appreciated. How do this places operate? Is each spring/pool/etc. a separate business? Attached to a hotel? etc.?