Sim Lim Square (map) is the place to be if you want to buy electronic items.
It's one of the two shopping malls in Singapore selling mainly electronics, the other is Funan The IT Mall (map). Both shopping malls sell the pretty much similar stuff, but Funan is more spacious.
If I'm buying computer spare parts, my preference is to go to Sim Lim Square.
Usually at the entrance, there will be people distributing price listings in the form of pamphlets for the different shops. I'll grab all of them and do a quick price comparison. Prices are competitive. I just bought an external hard drive and it cost close to what I can get on Amazon USA after shipping to Singapore.
Horror stories of customers being ripped off by highly marked up prices in Sim Lim Square are not uncommon. For that reason, you should always research on prices online before heading down. The prices should not vary too much from standard retail prices, usually I would write down the prices I find on Amazon and before heading down.
Some of the shops are quite notorious. They might get shut down because of complaints to the authorities like CASE. But they almost always resurrect by coming back with a different name for the shop.
For non-computer parts like cameras, TV, printers and others, I personally prefer Funan. You can definitely find the same thing at Sim Lim Square.