Hm. Maybe you're just not looking close enough to find something. I don't know many specific sources for cheap car heatercores (except for germany), but commercial PC-Radiators are not that hard to find, and most manufacturers over here in germany DO ship anywhere in the Euro-Zone, some even worldwide. You just gotta ask. I have to know that, since I work in the biz!
A recent review on PC-Radiators, including a car heatercore (labeled 'Airplex', originates from a french car) and the Black-Ice series:
http://www.hardwareluxx.de/cgi-bin/u...c&f=5&t=000038
Just go and search thoroughly, and you'll find more than enough; regardless if you want to build your rig yourself or buy serial parts.