I dunno about the best place, but there are a couple of options.
First off, I am going to presume that you want a Nintendo-compatible monitor because you are restoring a Donkey Kong or other Nintendo game using inverted video. If you are going to MAME it or put a non-Nintendo game in, the options are different and more plentiful.
Used EZ-20 monitors aren't too hard to find (RGVAC or eBay, etc) and I have had good luck with them. Bob Roberts, ArcadeShop and others usually have spare flybacks and cap kits if you need to fix it up.
If you want a new monitor to use with Donkey Kong, ArcadeShop has a converter board that is plug-and-play with a Vision Pro monitor (which they also sell).
Info for Bob Roberts (aka The Real Bob Roberts) and ArcadeShop can be found in the vendors section of the BYOAC wiki:
http://wiki.arcadecontrols.com/wiki/VendorsCheers