Welcome! There are many places to buy arcade parts. Are you building a perfect scale replica? If so, jakobud has drawings of the original that could be helpful:
http://jakobud.com/cabinetPlans.phpArcade parts can be bought in many many places. eBay, Ultimarc, mikesarcade, etc. If you're trying to use an original PCB - you'll have to search ebay, craigslist, or KLOV (a forum for arcade collectors). But, personally, I wouldn't put an original PCB in a scaled down cab. Because then you'll have to figure out how to properly power it, connect it to a monitor, etc. Depending on your skill level, I'd go with a PC running MAME or a 60- in 1 board or a pandora board or something. In my opinion, original PCBs are best used in original cabs.
What type of screen are you thinking of using? That can make a big difference in the rest of the design.