You can try surfing on ebay (www.ebay.com.sg), as in the Singapore site for ebay and see if there're people selling.
Or you can try Mumsfairy website. I dont't have the address off hand. You might want to google it.
You can also google for Moms in mind. These are the few companies I know that carry Medela products.
If you must go to a shop, you can try Mums and Babes. You can call them before going down to prevent wasting a trip.