Airpcap. It is the only card supported and it is USB (they don't have any PCIe or PCI versions). They are not cheap.
If you are looking for cheap, look up 'atheros pcie' on ebay and use Linux (such as Kali/Pentoo/etc).
If you are looking for cheap, look up 'atheros pcie' on ebay and use Linux (such as Kali/Pentoo/etc).