Hello,
The issue with Dell's is that the heatsink is mounted in such a way that it is held down by clips on the mobo. Now getting a after market heatsink will not mount this way and the way the dell mobo's are mounted there is no way to use a heatsink that mounts thru the mobo.
So your best option would be to see if Dell has a replacement heatsink.
Cheers,
Mak