Suppliers
There are many internet based mail supply companies that provide most of the hardware that is required to make your projects come to life:
Adafruit Industries (electronics)
Amazon (just about anything)
Amazon Supply (hardware)
Digi-Key (electronics)
FingerTech (robotics)
Jameco Electronics (electronics)
McMaster-Carr (hardware)
Mouser Electronics (electronics)
Power Supply Depot (power supplies)
SparkFun Electronics
(electronics)
Surplus-Shed (hardware)
Pololu (robotics and electronics)