if you're in the UK have a look at the special offers page on
http://www.hantarex.co.uk/ they have a number NOS monitors at very good prices.
i recently got a 21" video monitor from them. Whilst not stictly an "arcade" monitor (it has Scart, composite video, svhs and TTL/analogue RGB inputs) i have successfully linked it to an ArcadeVGA and getting a good display. plus the connectors make it easy to connect consoles to the monitor as well. using the Scart connection means no need for a video amp.
The only downside of getting a 21" monitor is that it will not fit in the std cocktail cab design. I will need to adjust the dimensions to ensure it will fit. it seems quite heavy as well.
I think the std cocktail design is for a 19" monitor (maybe a 20" might fit).
PS. note their EV (SVGA) monitors are basically PC monitors and may not what you are looking for to get the authentic arcade feel. I have an MTC EQ/2 video monitor (std resolution). Their Polo range are the gaming (arcade) monitor range, some models are fixed 15.7KHz horiz freq (std resolution) and also have multi freq models (medium resolution).