Vendors

Here is a list of vendors for reference.

FIRST specific

AndyMark  (Rev Robotics)

VexPRO (West Coast Products)

BaneBots

Bimba

Cross The Road electronics
Competition Robot Parts (CRP) 

REV Robotics  (Amazon Site)
RoboPromo Promotional (decals, banners) and bumper materials

General Hardware/Materials

McMaster-Carr 

Rankin Automation (8020 suppliers:  great discount to FIRST teams)

Grainger

Instructables

Bolt Depot

ePlastics
OnlineMetals
MetalsDepot
VXB (Bearings of many types.)
Vulcan Spring
Viair (compressors)

General Electronics

DigiKey

Mouser

Allied Electronics

Automation Direct (They have brass fittings, too)
Powerwerx

Mechanical (Gearing, etc.)

Stock Drive Products

BoltDepot

Bumper Supplies

Pool Noodles

Bumper Fabric (Get 1000 Denier Cordura Nylon, red and electric blue.)

Hobby Electronics

SparkFun  Tutorials 

AdaFruit
HobbyKing
DF Robot
16 Hertz
MaxBotix

Misc Robotics 
Innovation First International (IFI) 

Trossen Robotics

The Robot Space

Sensors
US Digital  (Encoders, etc.)
Kauailabs

Pneumatics