Kortek has/had a KT-3414DF model. I think the viewable size on that was 32-33". It's a larger version of the KT-2914DF that Betson sells. It was used in In The Groove 2 and later Pump It Up GX cabinets. However, PIU (ITG is out of production) recently went back to using 27" monitors, but they didn't give a clear reason. The rumor is that they could no longer get the KT-3414DF from Kortek. My PIU unfortunately has a different, VGA only monitor (I think it's a Tatung), so I can't provide much more info.
The KT-3414DF was CGA-SVGA, dynaflat tube. A very nice monitor, but I'm told it retailed for close to $1500, and you'd likely have to have it shipped from Korea if you can still find one new. There's a Kortek rep in the USA who may be able to locate one for you.
Oh, one other suggestion: if you're willing to accept interlace, you can do 800x600 on an EGA (medium res) display at around 72-75Hz. You can also do it on a CGA (standard res) display at around 50Hz, but that'll flicker really badly, and you'll also likely have smudgy edges as that'll exceed the analog bandwidth of most CGA monitors. Also, be aware that not all displays are specified for operation above 60Hz, so that may complicated the finding of a suitable medium res monitor.