If you want new, the answer I give would depend on what size and type you want.
For 19", check with Playdium Store (formerly Starburst Coin) -- they're a Happ distributor and here in Toronto (401 and Martingrove). I don't know if they are stocking 25" (or 24.8") standard res or not.
For other stuff (bigger or multi-sync), check out
Nieman Displays. Rick is in Cambridge and makes cool stuff.
If you don't mind used, I would post on CGCC or head to the auction. But, I would expect to have to install a capkit (replace the electrolytic capacitors) in a used monitor.
BTW -- a vertical monitor is just a horizontal monitor turned sideways, although there are some frames that are easier to mount vertically than others.