Have seen these dozens of times before, but not thought much of them until I needed to get my hands on some. What are these connectors called and where can I get some? Local hardware/retail store perhaps?
If you use standard rocker or toggle switches or wire up to relays like the picture above without using the standard relay connector, you'll find yourself using bags of these over time. We use them in places where we cut/split in to factory vehicle wiring to allow an easy tear down and restoral to normal operation prior to vehicle surplus sale.
Quality does go a ways with these - I prefer the 3M brand with the correct crimp die when used in an automotive application.