I don't know who is telling you that 255/35 and 275/35 are not available but they are. See the link below. You are not going to find a random tire shop that has them in stock in their store. Expensive performance tires typically need to be ordered.
Order them from tire rack and have them sent to a reputable tire shop to be installed. Tire rack is a supporting vender on this forum and they are a great company. I buy all of my tires from them.
EDIT - I just noticed you are from SA. Good luck tracking down the tires; Most of the people on this forum are from the US.
'13 SSII M3 Coupe - KW Clubsport 3-way, Akrapovic Evolution, ESS Tune, StopTech Trophy, IND Euro Front Bumper and Gloss Black Mirror Covers, TMS Studs/Lugs, BMW Performance Carbon Fiber Spoiler/Interior Trim/Center Console, BMW Edition Grills/Gills, BMWPedals Extended DCT Paddles, Advan TC3 18x9.5 & 18x10.5 - Hankook RS3 (Track), HRE R40 - 18x10 & 18x11 - Michelin PSS (Street)