Looking for something else, I stumbled upon this vendor of plaster statuary and casts especially for drawing. If this supplier has already been mentioned in this or another similar thread, I'm unable to locate it.
http://www.elementsofhome.com/catalo...d=6/index.html
There is quite an array, in about 9 different categories. I was surprised to find one anatomical "Horse" that I had seen this past weekend for the first time, in two different paintings, at an academic show.
It seems to be a Chicago enterprise. I'm puzzled as to why past searches hadn't uncovered this source. The prices are not unreasonable.
A caveat -- a good deal of the value of having such items as models for drawing or painting lies in your own options with respect to setting up the lighting. These all admit of a variety of "looks," depending on distant or close lighting, strong or filtered, one or many sources. This is just to say that it can be a little difficult to select just the "right" sculpture or cast from a catalog or web page. You just have to try something that looks interesting. Don't tackle intricate and complex forms straight away. However, when you're ready, the David's Right Eye, the David Bust, some of the hands, and the Homer and Tartan masks are a few of the studio favorites. I don't believe I've seen that Socrates Bust in real life, but it looks like it would be a worthy opponent.