What I could suggest:

Walk around to some recommended shops here on Sukhumvit. Rajas, Rajawongse, Crown. They arent TOO far from one another. Check out the men & women's prices then decide where you want to tailor your clothes.

Crowns for womens clothing are pretty good, as they are with men. But, as many people would agree with me, most tailors in Bangkok do better work for men overall. Dunno why that is, but it always seems to be the norm.