After using off-camera flash for awhile, it is sad to say that at this time, there aren't a lot of places to get the gear in Singapore, and even if there is, it is very limited in variety. For that reason, I have been looking at buying from places like B&H, Adorama or Amazon.
However, if you are just starting out, there are a few places to check out. I would strongly suggest that you do some homework and research because similar products can be sold at very different prices.
Here are some places to check out (in alphabetical order):
Cathay Photo @ Peninsular
Ruby Photo @ Peninsular
TagoTech (online orders)
The Studio Outfitters (online orders)