Rocker's jeans and leather/silver accessories- other than Far East Plaza, you can try Peninsular Plaza and Queensway Shopping Center. 77th Street is a chain of shop that carries what you're looking for. It has branches at FEP.
As for a leather jacket- in Singapore? Are you sure?

I have a leather jacket but I've never worn it out. Ever. Not in this heat.
If you're going for the rocker look, it should be enough to get the ripped jeans, black ankle boots and lots of leather/silver accessories. And of course the t-shirt. It's always about the t-shirt.