These are catalogs and websites that I review.

Fasteners

• McMaster Carr
• Misumi
• Stanley Engineering
• Böllhoff
• PEM and PennEngineering

Aluminum Extrusion

• Misumi
• Item North America

Motors

• There are many gearmotor integration companies that have standard offerings, and can make custom assemblies for you. The remaining items in this list are if you want to buy a small motor without talking to a sales representitive.
• AliExpress
• Hobbyking
• Misumi
• Robotshop
• Tower Hobbies

Electrical Prototyping

• Octopart
• Digi-Key
• Mouser
• Adafruit
• Sparkfun

Cases and Racks

• Pelican
• Hammond Manufacturing
• Takachi
• Phoenix Contact

Latches, Knobs

• Elesa
• Lee Valley
• McMaster Carr
• SouthCo
• TCH

Buttons, Human Machine Interface

• Adam Tech
• APEM
• C&K Switches
• CIT Relay & Switch
• EAO
• E-Switch
• NKK
• OTTO

Hydraulic Fittings

• Parker

Metal Stock

• Metals Depot
• Metal Supermarkets

Industrial Sensors, Controls, and Safety

• Allen Bradley & Rockwell Automation
• Beckhoff
• EAO
• OMRON
• Schmersal
• SICK
• Siemens

Cable Management

• Icotek
• Igus
• Phoenix Contact
• Panduit