I can recommend two sources for 120 folders. I have bought from both of them, and have been very pleased with the results.
The first is Pacific Rim Camera (
www.pacificrimcamera.com). Their pricing is fair, and I have found their descriptions to be accurate. I've purchased a Zeiss Contessa 35 (a 35mm folder with coupled rangefinder) and a Rollei 2.8F, among other gear, from them.
In particular, check out their Zeiss cameras (Folder and Pre-Folder). They have a Nettar on sale now that might meet your needs.
The other is the Web site of David Richert, who also repairs and sells repaired folders. His home page seems to have been hacked (just checked), but you can see some of the scope of his work here:
http://www.davidrichert.com/photography.htm
He also sells on ebay. I bought a Zeiss Ikon Super Ikonta (BX) from him, and he had done some work to overhaul the shutter and clean it up. Pricing was very fair, and condition was excellent.
With any folder from the 1950s or earlier, of course, you should probably factor in a necessary CLA, and possibly more (recovering; possibly redoing the bellows), to get the camera to perform at its best.