You need to find one with about the same thickness and dimensions, because if you find it thinner then it won't make contact with the VRMs at all.
I googled around and searched this forum for thermal pad recommendations and found this thread:
https://bitcointalk.org/index.php?topic=54617.msg702129#msg702129Phobya Thermal Pads in .5mm and 1mm. Seems like I might need the 1mm if I'm not replacing the memory pads (square pads?). Any recommendations or things to avoid?