There is a German website that is a good source for researching board part numbers. The boards are the products called 'camera module'.
It is more complex than just buying the boards though. You also have to separately buy the USB connector parts and cable if you are going to use it as a USB device. If you are still interested, this discussion is helpful.
Also bear in mind that the R200 and SR300 are about to be retired as the new D415 and D435 cameras are introduced, so boards for R200 and SR300 might be hard to obtain soon. So you should order a board sooner rather than later for those models.