Specialty sewing machine stores should sell them - I know my mom got hers years ago from one - it actually has dials and things to adjust the size/dimensions of the mannequin for different sized ladies, it's pretty neat.
Also, did you check E-bay? I took a quick look and found this - I'm sure you could find more though... not sure how much you're looking to spend, but they are quite costly.
http://cgi.ebay.com/2-x-Female-Manne...em330027339290
Good luck! Your daughter is quite talented- I saw your other post and the shirt she made!