Parts-People.com is a great resource for finding/buying Dell laptop parts and they also have guides on how to fix it yourself.
You're going to need one of the two fan models on the page below. Check which one is already in your system and then purchase the appropriate one.
[http://www.parts-people.com/index.php?action=category&id=148&subid=219&refine=heatsink|http://www.parts-people.com/index.php?action=category&id=148&subid=219&refine=heatsink]
Then use this guide on how to replace it.
[http://blog.parts-people.com/2011/04/13/dell-studio-1535-1536-1537-cpu-and-heatsink-fan-removal-and-installation/|http://blog.parts-people.com/2011/04/13/dell-studio-1535-1536-1537-cpu-and-heatsink-fan-removal-and-installation/]
Hope that helps!