I need a wall plug to insert my apple watch charging cable in. What do I call it and where do I get it?

an usb ac charger

apple mades two types an usb-a and an usb-c

Apple 5W USB Power Adapter - Apple

Buy 20W USB-C Power Adapter - Apple

which type you use depend on the cable you have for your watch, you should also be able to charge using other non apple ac chargers with the correct usb connector and even a computer with the correct usb connector

You could always use the one that came with your iPhone.

You can also plug it into a USB port on your Mac (just about any USB port will do)