For general retail: Code 75 FB rail is marketed by Peco, whilst C&L offer Code 83 only.
It might be worth seeing whether there's any other FB rail available from HO track suppliers.
The finescale societies are not much different in what they offer in the Members' Stores. The EM Gauge Society only has Code 83, which strikes me as a bit "tall", so I held off buying any at the recent annual EMGS show in Bracknell.
The Scalefour Society currently stocks both Code 55 and 82 FB. Code 55 would probably be ideal for portraying very lightly-laid lines but may not work very well if you don't use finescale flanges.
This weekend is the annual RailEx finescale show in Stoke Mandeville, which is a chance to see if any of the trade stands have suitable stocks of FB.