Where to buy camping tarp?

I know there's camping shops around in mong kok but last time i was there i didnt see any tarps. well to be honest i wasnt looking for tarps at the time so i may have missed them. anyways im in shenzhen atm and will be going to hong kong soon and would like to buy a tarp while im there. If anyone knows where i can buy a tarp it would be great, and any opinions on what kind of material i should buy would be much appreciated. Im thinking silicone nylon since ill be backpacking for a long time but its really expensive o.O thanks in advance