The list is certainly not comprehensive, so if you don't see your favorite site, gimme a holler and I'll add it on. The sites with a logo are sites where I've had personal experience buying. Repair and restoration links can be found here.
Comet is all about lighting - huge selection of LEDs and specialty bulbs and lightning mods. "Our company goal is simply to find ways to bring to fellow Pinheads a superior collection of LED products to enhance their personal and collections. This creative customizing results in outstanding visual enhancements for gameplay."
Marco's IS everything pinball with great customer service and super fast shipping. If they don't have it, they will tell you where you can find it. LEDs, playfield parts, coils, boards, electronics, cabinet parts - they have it all.
Steve Young is definitely old school pinball - and that's a GREAT thing. Steve and his team KNOW pinball. If they don't have the part, you are unlikely to find it any where else. Ordering is done via email or phone only - no website orders. They have many, many more parts then are actually listed on the site.
Coin Taker is another very popular site for LEDs, but they have much more. Along with great customer service they sell machines, mods, electronics, ring kits, and much more.
Pinball Life is a small family run company with a LARGE WAREHOUSE of anything you need to get up and running again, plus a whole lot more. Great pop cap lights!
Another great site for new boards, board parts, and board REPAIR! Drop Keith a line and see if he can get that board repaired before buying a new one.
Nabour's Novelty is a one-man shop that has been rebuilding pinball machines for the past 40 years and Tim is willing to share his knowledge and help any way he can. Great source for board repairs.
Other Great Part Sites with a few notes on key products (just click on the name):
Big Daddy - Electronic parts & repair kits & Alltek boards
Mayfair Amusements -phones calls or email only to order
Mr. Pinball - Australian site
Passion for Pinball - the site for the famous Cliffy's protectors
Pinball Center - UK site but ships to US; sells light up posts
Pinball Decals - decals and clear washers for protecting playfield plastics
Pinball Pimp - restorations, sales, and cabinet stencils
The Pinball Wizard - parts & repair & Rottendog boards
XPin - Displays, power supplies, & boards
Repair & Restoration Links (just click on the name):
"I've been repairing pinball games since the early 1990s. In that time I've written and documented more on pinball repair than probably anyone else on the planet. From the pinball repair guides at PinRepair.com, to seminars at all the major pinball shows across the country, to writing and editing for pinball magazines, to the subscription pinball repair site PinballNinja.com (with over 800 game repairs documented including over 400 with HD video), to DVDs on pinball repair (pinrepair.com/TOP), well you get the idea. I am also inducted into the Chicago Pinball Expo Hall of Fame in 2010, and received a pinball lifetime achievement award from the Pacific Pinball Museum in 2011 for pinball repair."
"This is an introduction to pinball maintenance and simple repairs for new pinball machine owners. There are many repairs and checks that any pinball owner can do without any expertise. If you are a new owner or thinking about purchasing a pinball machine, then this site on pinball repair tips is for you."