BNS Speed has pretty good selection. Sometimes
mxhousethailand is a good address to get gear at good pricing. Another one I used to go is
Helmet7shop – ความปลอดภัยที่คุณเลือกได้ opposite Suriwongse Book Centre.\
Nowadays I mostly order from abroad, because I cannot get my size here in Thailand in most cases. Also on FB are some good groups for second hand gear from the Thai community not my first choice but, if you cannot get your size, that's a good way to get you covered. Parts I get mostly at Saengchai and sometimes from China or Indonesia.