Where to buy parts online?

Hi all, I was wondering where to buy parts for PowerBook G4 15" HighRes?

I am looking for left and right fan and maybe a heat sink.

What is a heat sink for anyway? The reason for this is that my laptop has been overheating, and feels burning to the skin. Not really usable as a 'lap'-top anymore. It is way too hot.

I think the fans are a problem.

These sites might have what your looking for:

Apple Powerbook G4 Parts for Repair - Aluminum 15 inch

PowerBook G4 Aluminum 15" Parts - iFixit