General hardware; screws and bolts: McMaster-Carr McMaster-Carr
Electronics parts; jacks, pots, capacitors, etc.: Digi-Key https://www.digikey.com/
Excellent vendors. To general industrial supply I’ll add:
https://www.mscdirect.com/
For electronics I’ll add:
https://www.mouser.com/
Allied Electronics - Fueling the Future of Service Station Automation Today
Antique Electronic Supply
All are in the US.
For circular saw blades and router bits I’ve had good luck with:
Amana Tool® CNC Router Bits, Saw Blades | Power Tools & Supplies | ToolsToday but there’s so much internet competition that I’m sure there are lots of other good options.