These are all sites I personally have used in the past, and have had excellent results with.

Maker Sites

Adafruit Seeed Studio Spark Fun Maker Shed Ponoko

Cables

Electronics Components

Jameco Digikey Mouser

Computer Sites

Newegg Frys Tiger Direct PC Part Picker

Suggested by Friends

Sites here I haven't personally used yet, but were recommended.
Surplus Gizmos Element 14 Allied Electronics Newark Electronics The Big Bearing Store O-Rings USA McMaster-Carr Grainger Motion Industries Amazon Supply