Well, it's not quite arcadey, but you could get a Playstation 2 controller and a USB adapter for it (they sell a great one at Radio Shack). That gives the d-pad for 4/8 way games, an analogue stick for games like Star Wars, and a second stick for Robotron/Smash TV and plenty of buttons.
However, if you are building the rest of the cabinet, it's not really much more work to make an external controller box with actual arcade controls. Get yourself a Ultimarc Ultra-360 joystick, wiring harness for the buttons, and a set of buttons with switches. Then build a box with a drilled hole for the joystick and holes for the buttons (they all can use the same size hole). The joystick just screws on from underneath with 4 screws, but buttons are even easier, just stick them through the holes from the top and screw on the button nut underneath. Then attach the wiring harness to the joystick (it has a bunch of pins sticking out, the harness just plugs into that), attach some quick-disconnects to the ends of the wires, and attach those quick-disconnects to the button switches. The switches just snap right into the bottom of the buttons. The joystick just has a USB cable coming out of it, so all you need is a hole in the back of the box for that wire to go through (and maybe a longer USB cable / USB extender), and voila! One laptop control panel.