Hey Dan
I've been searching for horses myself, Verlinden does a bare horse I & II in 120mm.
Bill Merklein is casting great 54mm horses at $25 ea., I know he has done a Galloping Cleveland Bay, Napoleons Arabian, a shying Standard bred and a walking Andalusian in his first series. Here is his contact info. Bill Merklein, 156-04 71st Avenue, Flushing, New York, NY, 11367. A review of some of his work is in Military Modelling (U.K.) Vol.34 No.6, May-June 2004.
I'm using a bare horse from the now out of production Fort Duquesne in 120mm which is very nice and worth trying to find!
Historex does nice work and they are availble from Squadron, although I have yet to find two figure painters who agree on their size (too narrow or too wide).
I've been eyeing the 2 available from MR Ltd as well, both nice peices.
Thats about all I've come up with, I'll follow this post since I would still like a few more 120mm horses.
Cheers
Mark