Try 888 bike shop, on old San Kamphaeng road, just east of middle ring road. Closed on Tuesdays.
I'm assuming you're in Chiang Mai as that was mentioned in another reply.
https://www.google.co.th/maps/@18.7787258,99.0494349,3a,75y,227.24h,90.23t/data=!3m6!1e1!3m4!1syt6QcWFuJMa7-GwDZLrUNg!2e0!7i16384!8i8192?hl=en-GB
Before buying parts, you need to know what kind of rear hub you have.