They are beautiful mugs and I can easily see how people become hard core Fire King Mug collectors. When you get a chance do a completed items search for Fire King Mugs and sort by highest price. You can see all the cool rare Fire King mugs selling for top dollar. (Snoopy, Pizza Hut, Nike, Ghostbusters, McDonalds and many more)
So keep an eye out at Thrift Stores, Garage Sales, Craigslist.