Just walk into a hardware store and buy a sheet of wet'n'dry 250 grit, 400, grit and maybe 800grit if you really want?
People will argue for days about how high u should go, rough vs polished finish etc. I had tested them all and I am finding that 400grit is a smooth as I need it, quick as well as if u get really into it you can spend a hour or more performing what is really a simple task.
Your just trying to get the base of the heatsink flat, not turn it into a mirror. As always its worth practicing on some junk before you attack your new heatsink.