Metal: Industrial Metal Supply: www.industrialmetalsupply.com
Wood: Woodworker Source: www.woodworkerssource.com
Plastic: calsak www.calsakplastics.com, check for scrap
Plastic: There is also new/old company at 2570 N Huachuca Dr, check for scrap
Homedepot: https://www.homedepot.com
Amazon www.amazon.com
eBay www.ebay.com
Sparkfun www.sparkfun.com (Makers, Tutorials)
Adafruit: www.adafruit.com (Makers, Tutorials)
Digikey: www.digikey.com (Commerical)
Newark: www.newark.com (Commerical)
Monoprice: www.monoprice.com (Cables and more)
PCB: Advanced Circuits http://www.4pcb.com/
Grizzly www.grizzly.com (Tools)
LittleMachineshop www.littlemachineshop.com (Hobby metal and wood worker)
Harbor Freight www.harborfreight.com (Tools)
McMaster-Carr www.mcmaster.com (Hardware)
AliExpress: www.aliexpress.com/
Banggood: www.banggood.com
AndyMark: http://www.andymark.com
VexPro: https://www.vexrobotics.com/vexpro
West Coast Products: http://www.wcproducts.net