Bluetooth Remote for Mac (for Plex)
The Satechi Bluetooth Smart Pointer is actually a pretty good choice. I have a number of teachers at this school who use it for their classes all the time. It works a lot like the Apple Remote that has been (essentially) discontinued for Macs. It has some additional features, like a laser pointer. To make it work with Plex, you can just use it in Accessibility Mode (this makes it behave more like a keyboard than a remote).

On a slightly unrelated note, it can also be used to control iOS devices, which I thought would be useless; but, it turns out to have some very convenient implementations around campus that I hadn't imagined. 

Certainly worth $50, from my perspective.