Here are a list of some useful sites:
DIYChristmas.org – My favorite site to use. Not the largest community, but VERY friendly and helpful. This is the best forum for mechanical help/talk.
doityourselfchristmas.com – Probably the largest community and one of the top places to go for help with your pixel projects. My second favorite site.
PlanetChristmas – One of the originals. Not as active as it used to be, but still some good people use it, and there is a lot of good old stuff on there.
hauntForum – Halloween based, but a great source of mechanical projects and insights. Since Halloween was neglected for so long, these folks have been having to make and rig up props longer than us Christmas guys.
Electronic Supply Sites
eBay – Lots of great parts to be had. Some come from China, so it could take a while, but the prices are amazing (and with free shipping usually), just watch out for the seller’s feedback rating.
MPJA – Great site for surplus electronics
Tayda Electronics – Great prices on everyday needed parts. Shipping is a little slow (like a week longer than you would want) since they seem to ship their stuff from China to them, then to you. Worth the price if you can stand the wait!
Surplus Center – Great site for all things mechanical. Really good prices and great customer service.
xLights – Amazing free software that is WELL maintained and is used for sequencing your pixels and creating a schedule if you wish.
Wiper Motor Page – The ORIGINAL source for all info wiper motors. Still useful for good background information.