There are a few reasonable tailors at Far East Shopping Centre on Scotts Road, a few doors down from the Hyatt and more or less right across from the Royal Plaza on Scotts Hotel.

The outfit I can recommend is Christian Armani Tailors (sounds really dodgy, I admit) which you can reach at +65 67336407. They do acceptable quality clothing for a more than reasonable price. It does help to know exactly what you want though. Oh yeah, and the more you buy, the better the price you will get.

Good luck, and have fun in Singapore!