Being a Thai citizen, I was told that Dioni located around Sukhumvit 18 is a great place. Many well-known Thai figures come to get their suits made.
Another one is called Savile Row located at Rose Hotel. It is somewhere in Silom Road. This place is quite popular amongst expats who live in Thailand.
As far as I'm concerned, both of these places have Zegna, Dunhill, Burberry, Versace, and others cloth to choose from.