I get most of our electronic parts from http://www.mouser.com and http://www.digikey.com
Tools and general stuff (a LOT of stuff) cheap from http://www.zorrotools.com but you have to work to find things, their search is less than optimal.
Tools http://www.use-enco.com , there's often discount and free shipping codes available.
Various parts/materials at http://www.mcmaster.com
Filament and stuff http://www.octave.com , http://www.makergeeks.com, http://www.homedepot.com (really!), http://www.amazon.com , and others.
Good beer at http://www.bevmo.com .

Feel free to add other suppliers you know of for parts/add-ons/whatevers.