Maximum Force, Area 51 and Area 51: Site 4 all use the standard Happs four pin interface straight into the PCB. They pop up on eBay every once in a while, but it seems like not that many people want to trade or sell them.
Though if you end up with a single board Area 51 game, I can sell/trade you the chipset for Maximum Force. You can make your own hard drive with the MAME chd images.