Try Raja's Fashion (near Nana hotel/BTS) or Jhasper's Fashion if you guys are heading to BKK. Will take approx. 4 days inclusive of fittings/alterations though.

Raja?s Fashions in Bangkok, Thailand - Lonely Planet
Time stated is inaccurate. Do go only after 11am and avoid CNY and peak periods like the Queen's B'day in the middle of August.