Coming from Europe, I am used to radiator heating systems and thinking about putting one into a fixer-upper house I intend to buy. Where can I find such a system in the North-American market?
You should be able to find them in any of the original colonies in colder climates, as they're needed for restoration work in older homes. (so, eastern seaboard, from Maryland to the north)
American radiators tend to be heavy steel or iron and gravity fed, with larger pipes that I don't believe are as efficient as those found in areas like Germany. My neighbors are currently going through a remodel of their home (1940s construction, in Maryland) got theirs from an architectural salvage place. The burners (oil in their case, and in my home), you can still get new so are easier to replace.
Contact your local plumbing supply store; if they can't tell you where to find radiators, their customers (plumbers & pipe fitters) might be able to.
You might also find the 'baseboard heating' where there's a copper pipe with fins attached that run along the whole wall of a room. If you just want the non-drying qualities of radiator heat but can't find radiators in your area, this might be an alternative. (I've never seen fan-assisted models in the US, either, but I'm guessing if they have them, it'd be in the northeast.
Typical early style radiator heat is nothing like modern European style radiators. You should find out what originally came with the house to make the best fit. There is steam heat, water heat. The boiler may be powered by oil, natural gas, or (if it's really old) coal. As Joe mentioned, the radiators can be gravity powered or propelled with a circulation pump. The gravity fed systems use large cast iron pipes down to the boiler and is less efficient. A circulation pump feed system uses smaller pipes and is more efficient, but you probably wouldn't want to retrofit a pump on a gravity fed system without changing out the pipes.
I've seen a This Old House episode where Tom placed a European style thermostat on a standard water filled radiator. So, this may be an option for you.