I dont know where to get the push pins specifically, but have you thought about simply bolting the fan down to the card, and using a couple of those little rubber grommets as vibration dampners?
Could even go as far as using butterly nuts (wing nuts) to tighten up, giving you more control on the pressure you apply.
i was unsure if the metal contact would be an issue?
I'd use Nylon screws to be on the safe side.
Put me on the spot now, even the slightest sniff of a hint is not allowed
email address in his trust *wink*