Certain motorbike gear can be bought in Chiang Mai but European standard gear will cost more than in your own country!  There are many manufacturers (mostly Chinese) that sell motorbike gear here in Thailand and to be fair for the price most of it is actually good quality.  jackets and Trousers, knee pads, arm pads etc can all be bought in Chiang Mai however please make sure you arrive in Chiang Mai allowing yourself plenty of time to buy anything you may need.