There are several manufacturer's for display mannequins in the United States.
Several of these advertise in Museum News.  Another source for mannequin
manufacturers may be found in the AAM's Official Museum Directory 1998, which
includes porducts and services listings for the first time this year.  A
cheaper alternative would be to use discarded store mannequins, However,
mannequins of this type can be problematic for the display of period (and
fragile) costume.  Good luck!