If you google heat sink paste you'll find loads of products. If it's the sort of heat sink that clips on securely, the paste will fill any gaps to give good heat conductivity. However, the paste is not a very efficient adhesive - vibration or a heavy landing may dislodge the heatsink if it's not a secure clip-on fit. You could try Maplin or similar for a double sided adhesive thermally conductive tape.
Or just use small strips of ordinary double sided tape at each end of the heatsink and inject the paste into the gap.
Ranavalona is spot on! Thermal paste or thermal tape. Go to your local PC store, they should have it. Take a look at PC modding forums, sticking heat sinks onto hot stuff has been a topic of debate for years with one general concensus: no-one agrees on the best method.