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.
'04 E46 M3 - Jet Black - Sold
'08 E90 M3 - Alpine White - Sold
'13 E92 M3 - Silverstone II - Sold
'13 E92 M3 - Alpine White - 1 Option (DCT), Ohlins TTX, AP Racing, Cobra Seats, Akrapovic GT4, BBS E88, Volk TE37, M3 GTS - Steering Wheel, Aero, 1/2 Cage, Seat Delete