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 (Eibach Front Spring Conversion), 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, Mode Carbon GT4 Splitter, Advan TC3 (18x9.5/10.5), BBS E05 (18x10/11), G-Force Rivals (275/35/18, 295/35/18)