I've been through this very exercise nearly two years ago for my 26 tourer. I did some research beforehand, and came to the conclusion that Bergs in Illinois was the place to get one from. There have been quality control and after sales support issues with another brand, plus the other brand costs more.
The "other brand" is imported into Australia, which may sound convenient, but even with postage from the U.S, the Bergs radiator was about $400 cheaper.
In view of the superior cooling from the flat tubes, that's what I ordered. More surface area is exposed to the passing air than with round tubes.
The Bergs radiator is a copy of a type once produced by the well known Ward company in the U.S. It has a very strong reinforcing bracket down both sides of the radiator, and I think more ruggedly constructed then the original. Bergs were very easy to deal with and it was no problem ordering one. It arrived very well packaged, even bolted to a wooden frame to protect it. As for performance, I've never been able to make it boil and every drive home for me ends in a 25km up hill trip.