I basically discussed this on the link below ....
... but the story is that I was just lucky. When putting the bike together I was on the lookout for similar frames and was aware that some FSRs of this period came with a sharkfin attachment enabling a disc to be fitted to the rear. It basically fit between the left side brake boss and the redundant hole at the bottom of the seatstay. I kept searching on Ebay for 'FSR' in the hope one would turn up or at least there be a donor bike. However I struck lucky when someone was selling a Stumpy FSR frame with the original Seatstay and Link together with the BETD Link and Disk Seatstay. I managed to get the frame, scavenge the parts together with the upgraded bearings and then sell the frame back on for around the price I paid for it. Lets just say the original price for the BETD parts would have been around £300 combined and I effectively got them free. I'm not sure the Seatstay is made anymore, certainly not on the BETD site, however one guy on the thread above say Risse do a whole back end for about $500.
Again I was just lucky, at the time I was just going to upgrade to XTR V-brakes if I could have got them for the right price.
2007 Trek Soho 1.0 Hybrid
1999 Cannondale R500 CAAD2 - Restored
1999 Specialized Stumpjumper FSR XC - Restored