Heat sink would be the metal heat spreader under your fan, if the fan does not even work now or not well, I'd just replace the whole thing. You would want to search ebay for a part for your laptop, would be cheapest. http://www.newegg.com/Product/Product.aspx?Item=N82E16835100014
would be good for the compound to add to the gpu for the heatsink.
It can be tricky to replace these things on a laptop, may want to find some help if you have never done this before.