ARCADE PART RETAILERS AND SUPPLIERS
Below are some good sites for finding arcade parts.
http://www.classicgamerooms.com/ - Located in Atlanta. Ships usually within 1 business day. A good source for buttons, joysticks, cabling and multigame boards. Also sells monitors, fixes XXXX-n-1 Hard Drives. The owner, Jason, is a really good dude though very busy!
https://www.paradisearcadeshop.com/ - Even though it is in Hawaii, shipping for USPS is pretty quick (priority times are honored). This is a good location for multigame boards and Hawaiian Chocolate candies for your significant other, which goes along way to settle concerns on how many arcade boxes you get in the mail each week. :)
http://www.arcadeshop.com/ - All sorts of classic arcade, cocktail table and parts. Site is a little old school, you will probably be ordering on the phone but they do ship quickly!
http://www.twistedquarter.com/ - Decent selection and a modern web interface with online ordering. I'm a fan of them for locks and various parts, not had good luck with their cap kits.
http://www.therealbobroberts.net/ - Seriously old school site but the basically the defacto source for monitor components, cap kits and other low-level electronic components. You will pay, my mail, with a check or cash. Yep, for real.
http://na.suzohapp.com/ - A very respectable catalog, reasonably fast ordering and shipping. Prices are sometimes steep.
http://www.digikey.com/ - A very nice source for low-level electronic components. I buy capacitors, transistors, resistors and voltage limiters from here all the time and they ship quickly.
PINBALL PARTS AND SUPPLIERS
Unfortunately there aren't alot (any, really) arcade and pinball specific resources but there are various companies that may support aspects of your repair / restore projects.
http://www.spectronics-inc.com/ - Located in Mobile, Spectronics has a very good assortment of low-level electronic components. They also offer board-level soldering and limited diagnostics.
http://www.communicationstoreinc.com/ - in Robertsdale, formerly Radio Shack though in the back of the storefront, offices with telecom and electronics professionals. They can do limited board-level solder work for you.
LA Showers & Glass - Spanish Fort, Alabama - (251) 625-4568 - The owners are arcade-interested folks and darned good folks. Their prices are reasonable and I've found them to be a great source for glass to cover arcade monitors and custom glass, plexiglass and acrylic cutting.