I use PoE extractors to power a few different devices in my house, including RPis and some non-PoE switches. It's ridiculously easy to use them, but you generally need to know the voltage you want ahead of time.
I got some POE adapters for my Google WiFi points so I can place them around the house and not mess with an extra power cable. Works great. You can get them on Amazon with various outputs (USB, USB-C, barrel connectors of various types).
In my experience, cheap ones on e.g eBay will give you 10/100, but I don't think they typically support gigabit.