I'm using a prism54-based mini-PCI card that I got on some web store, I don't remember which, but yeah, any mini-PCI card will do as long as you've got the drivers for your OS. Your question is really about antennae; I got mine from these guys: http://www.netgate.com
According to an invoice I just dug out, I got the following:
1 x 5 dBi Rubber Duck Omni RP-TNC (ANT-5OMNI-RPTNC)
1 x U.FL to RP-TNC Pigtail (PIG-UFL-RPTNC
You'll obviously want whatever kind of pigtail mates up with the socket on your card.
On my GCT, at least, there's a mysterious little door on the right side that doesn't really lead to anything that would be useful to an average end-user, but which was the perfect place to mount my antenna -- I just dremeled a hole & mounted the socket & then snaked the little cable in to the mini-pci slot, which is just a few inches away from the door.