The best bet, in all honesty, is doing it yourself. I'm not being an asshole here, truly there's nothing quite as satisfying as restoring an arcade cabinet. You haven't said what was wrong with it, but if you look around mame and arcade boards I'm sure you'll find all the stickers, marquees, buttons, and panels you'd ever need. If it's just artwork, you can always take the images to a local printing press. Usually anyone doing silk screenings, vinyl work, rubber stamps, they have the equipment to duplicate what you need.

If you're looking for new joysticks or buttons, there's a few online retailers that are reputable. If the monitor is out, you just need to find an appropriate tv that accepts RGB hookup. If it's the speaker, well providing it isn't the chip on the data board, that's usually one or two wires.

If the cabinet is messed up it depends on degree. Would sanding fix it? If you need a new cab cheap generic ones go for around $100. Depending of course.

You might have luck finding a local arcade operator and asking him who he buys from or uses to fix machines.