In this video I show how to make an inexpensive power drop that's great for use on stage and general use around a shop or rehearsal space.
I've never purchased one so I did some searching online today and was shocked to see how expensive these were. $100 for the drop and $100 for the cable on some sites. You can build the cable and drop combined for $40 using this method and these are incredibly safe and reliable when constructed correctly.

I would recommend going to a commercial electrical supply distributor that would normally sell to tradespeople. You will be able to find better quality parts than the big box stores and the cost will likely be less but not by a lot. Either way a hardware store will also have these parts. The power connectors like the PowerCON NAC3FCA and NAC3MPA-1panel mount can be found at www.parts-express.com

PowerCON male connector
https://www.parts-express.com/neutrik...
PowerCON chassis mount connector
https://www.parts-express.com/neutrik...

Parts Express also has the 4-40 machine screws you'll need but they do not seem to have the nuts with the external tooth lock washer. These make life much easier! Especially since it will be difficult to get the bottom screw in place if you have big fingers. You can find 4-40 external tooth lock washers from Fastenal or even Amazon.

4-40 External Tooth Lock Washer (package of 100)
Fastenal
https://www.fastenal.com/products/det...
Amazon
https://www.amazon.com/Stainless-Lock...

4-40 machine screws
Fastenal
https://www.fastenal.com/products/det...
Parts Express
https://www.parts-express.com/4-40-x-...