I go to these places when I want to (window) shop for guitar-related stuff:
1. Swee Lee - either at Bras Basah Complex (city) or Sims Drive (closer to where I live).
2. Excelsior Hotel - right beside funan. Has about 5-6 guitar stores. visit Davis Guitar and Guitar77.
3. Parklane Shopping Mall - about 4-5 small music stores, and one 2nd hand shop at the 3rd floor (By Left yata yun name).
4. Peace Center - right beside Parklane. Citymusic is at the 2nd floor of this mall.
5. Plaza Singapura - Yamaha at the 5th floor (or 6th, not sure).
Or, you can check the classified section of www.soft.com.sg.