I use a guy who has a stall at a market outside Tesco's in Saphan Mai. I've used him several times without any problems, and he always gives a good service. My 78yr old mum came over a few weeks ago and bought glasses from him. She has a complicated prescription, and normally pays 240 GBP for her lenses alone. He did her some Hoya lenses + frame for 6000 baht. She is very happy with them.