Some time ago, RF boards were available from Elitekeyboards.com, but they appear to always be out of stock.
They are now available from Amazon and eBay. Amazon has both the original RF87U and the new R2 boards. They sometimes have the PFU special edition, which has silienced Topre switches and all-45g weighting.
In my opinion, the best and most reliable eBay vendor is "Widebasket" (based in S. Korea). He has the Leopold version of the RF boards. As I understand it, the Leopold-branded RF boards are functionally equivalent to the Topre-branded ones, but with a different color scheme for the keycaps.
You can get more detailed information about models and model numbers by going to the Topre web site.