Few things about tailors in Thailand.
Tailors in the west will only tailor clothes for you from fabric that you purchased from them. In the east some tailors would accept fabrics. It is recommended you buy the whole suit from your tailor including the fabric.
You must ask them to recommend something that they would say would tailor nicely, not just blindly get the highest count that you can afford.
Get a wool suit while in Bangkok. Wool generally makes a good suit. It drapes nicely, tailors well and looks formal and nice. Contrary to popular belief, light weight wool is even good for the summer and in most cases better than polyester blended fabrics.
In Bangkok the famous among the local expats would be Excelsior tailor.
A must visit shop by the delegates and embassy staff in Bangkok.
115/1 Suriwongse Bangrak Bangkok 10500 Thailand
As an expat I would say they are infamous more than famous. Very nice untill something is not quite right, then they play games and string you along as they know your time is short. And why must they have young guys touting outside if they are so good?
Is this your business?