Heat-sink grease is very seldom used nowadays..
The excellent alternatives arenon-silicone-based pads and patches (
http://www.lightfab.com/products/gas...rface_pads:209) the thermal conductivity is the same or better, but since this is not a grease, it is clean :-) (It is unbelievable how that ugly white grease manages to be on everything I touched, it even finds its way to things I have not touched :-) )
You ca try any computer h/w shop, they are usually delivered with every processor yo buy. Once used (compressed between heat-sink and CCD or whatever you are cooling) you can not re-use it since it will be damaged too much, so you have to apply a new patch.
Good alternative could also be araldite mixed with metal powder, or electrically conductive paint (sold in spray bottles).
And, if you can not find anything from above, just silicone rubber (non-acid type) or silicone grease can be used (very thin layer will have good conductivity and the important thing her eis to remove the layer of air between the two surfaces.. Air is very good isolator (when it is not moving.. and it does not when in thin layers).