Hopefully a comprehensive guide on where to buy bot parts from.

Australian Stores!
BotBitz - The Australian Combat Robot Specialist store, kits, speed controllers, switches, radios and more
Plaig Bearings - A whole range of bearings, including those really odd sizes (2x5x2.3mm anyone?)
RS Components - Gears, belts, pulleys, bearings, bolts, nuts, wheels. It's Bunnings, but better!
Naismith - A large range of belts, pulleys, chains and sprockets.
Richmond Castors - A large range of rubber wheels to suit Featherwieght - Heavyweight bots.
HobbyKing - While actually a China based online hobby store, they have a warehouse in Australia. They stock everything from batteries, radios, motors, etc to general hardware.
Little Bird Electronics - Online electronics store, plenty of odds and ends for smaller bots
Jaycar Electronics - Store based electronics store.
Motion Dynamics - Online retailer with a large range of brushed DC motors.


International Stores!
Bang Good - They have a lot of stuff, brushless motors, radios, etc.
Maytech - Great range of brushless motors and speed controllers, including VESC.
Banebots - Large selection of motors, gearboxes, wheels and hubs.
Fingertech Robotics - The Canadian one stop shop.
Technobots - A UK store with a lot of hardware for building bots.
_________________
https://www.halfdonethings.com/

vxb.com has lots of bearings for pretty cheap if you are ok with international cheap and lowish quality
_________________
Mechanical engineers build weapons, civil engineers build targets