In Singapore the standout pro photo store is Cathay Photo
- reliable and helpful, unfortunately they don't list prices online. Email them.
Yes, those guys are very helpful.
I recommend the shop in Peninsula Plaza, though, since that's the bigger one. But they'll call up their shop in Marina Square for you to check if something is in stock there. Just be sure to ask if whatever's in stock has been reserved by another customer or not (I guess it's a cultural thing).
You should also check out the camera stores at Sim Lim Square in Singapore, it's possible to get very good deals there, if you're willing to walk back and forth and barter.
Remember to pay in cash if possible, that way you can chat down the price with another 3-5 percent (they pay a premium to credit card companies).
Also make sure that you get a completed GST return form, so that you can get a refund for those 5 percent at the airport!