One of the easiest and cheapest ways to get a new one is to call a TV repair shop and ask them to cross-reference the model number of the tube you had (Usually it's on a label on the back of the tube.. mine were both RCA tubes 19VMNP22 and 19VLTP22) to any of the used tubes they have lying around. I did this to a few places before I found one that would actually sell a tube to someone that wasn't a "Qualified Technician", it cost around $50 per tube and was much easier than getting a completely new monitor.
Also, ask them if they will leave the Yoke and the convergence rings on the tube. Even if it cost you a bit more. Moving the ones over from your old tube is a REAL pain in the ass. I can't even begin to describe how long it takes to line up the convergence rings if they are out of wack.

Got any useful ideas or tricks that you'd like to share?
niobium@arcadezone.org