Electronic Recycle/surplus shops like http://www.weirdstuff.com/
Go work at a hospital like me. I managed to get mine out of our electronic recycling center
+1Check the city streets on garbage day.
In Belgium top quality RGB (!) CRT's can still be found very easily. I've bought a B&O MX4000 two years ago for approximately 20$.
Since as Europeans we've been condemned to import most of our gaming goods in the past (in order to not be forced to play watered down PAL 50hz ports) surely we can boast a little about the great CRT's here that nearly always came with native RGB inputs?
If contrasting markets (and sheer availability of nice to have retro gaming goods) is to be considered irrelevant in the context of this thread, my post was irrelevant.