A friend recently went to Singapore and took the opportunity to visit Sim Lim Square. Sim Lim Square is the Technology building in Singapore.
The first store they came across that had huge selection of gaming peripherals as well as some great looking custom demo systems was Extreme Solutions.
The second store was PC Themes which had a fantastic selection of Peripherals and DIY PC parts. Like many stores in Sim Lim Square they will assemble your system for you.