Hello Luke, and welcome to Airliners!
There are a couple of things I do to monitor "interesting" aircraft, and both involve a site called Flightaware
First, create an account there. Then, you can sign up for "alerts" that will notify you when your favorite tail numbers depart/arrive/etc.
This, of course, doesn't work for airliners that use flight numbers, but it's a decent way to find other flights.
Also, you can monitor the Flightaware page of your home airport, and watch for interesting aircraft enroute.
My favorite Flightaware feature is the "Browse by Aircraft Type" page
. It lists all the unhidden civil aircraft in flight at any given moment. Why do I like this page? Because it's sorted by the number of each type aloft. That means you can scroll to the bottom of the list and see what single examples are in flight. I've seen the NASA B-52 in flight, as well as their Super Guppy, a C-133, an IL
-76, a B-17, and even F/A-18s.
Hope that helps. Once again, welcome to the site!