This info comes from years of emails that I've saved. If you have something to add or correct, please let me know. I can't verify all of it on an ongoing basis so some of it is bound to be outdated.
HE (Germany) has 320mm semi-floating brake disk, mounting ring and caliper adapter. 225
Stock BMW R100GS floating rotor 34 11 2 311 198
Stock BMW R100GS solid rotor 34 11 2 310 143, about $190.
Spiegler offers conversion of existing R100GS rotor for $220.
TAW (888-235-0910) carried Brembo and Discacciati rotors, $220 each (complete) for the K75 in 285mm size and they can get Discacciati in 300mm for $250. However, they don't have a caliper relocation kit as would be needed for the 300mm conversion.
EBC - standard floating rotors for about $200
Stock BMW rotors, $188
Galfer rotors: $190 each.EBC: $160-ish last time I knew (these were what I bought before).
Many different versions for R1100 models by Grimeca (from Motobins)
Braking USA has a dead website.
Brembo has little of interest on their website, but lists distributors.
Discacciati - can't find a website for them.
EBC - rotors and pads for just about every Beemer (no 320mm EVO rotors in 2003 catalog).
Ferodo - you can look for real information here, I gave up.
Galfer - website at first indicates they have no BMW products. Not so.
PFM, available in the US from EMA
Spiegler - Very good website!! Brakes for Airheads and older K models. No K11 or R11. Conversion of existing rotors for about $220. Wuedo used to sell their components but I think that is no longer true.
Wuedo sells Ferodo parts for Airheads and early Ks.
Perry BMW Exchange, (765) 564-6338 sold rebuilt rotors for about $90 as of 1996.