Unfortunately, you'll be hard-pressed to find any new CRT TVs except in places like India, Afghanistan, China, etc. Most retail stores finished phasing out new CRT sales about two years ago.
As far as used go, you could always check the local mom-and-pop thrift stores. Or you could score a better deal by checking Craigslist in your area. If you live in a large urban area, there are most likely several CRT TVs listed in your area under the "Free" section every day in your area.
But if quality and video fidelity is what you're looking for, get your hands on a professional video monitor like I did. Unless you get lucky, chances are you won't find one in good condition for under $50 unless you run into someone who doesn't know what they're selling. Sony's BVM line of Trinitrons is the gold standard, and most of their PVM line is pretty good too. For these you'll usually need a BNC-to-RCA adapter for composite video and of course a demodulator/tuner of some sort for game consoles that only output RF.
edit: Notice I didn't mention any websites. That's because you'll end up spending more on shipping costs than you would spend on picking up a CRT locally!