I've never seen an LCD that supports 15kHz RGB modes either.
Ones that have TV (composite video) input would certainly work, but I'm not sure how much the image quality would suffer using composite instead of RGB.
I use a 15" Mag monitor here on my A4000T, but runs through a PicassoIV which scandoubles all native 15kHz modes.
Beware of some monitors, like early Philips, which refuse to display scandoubled 15kHz modes, even though the input signal is within the monitor specifications. I'd suggest you try before you buy.
