Ive done 3d dress form cake for clients but would like to do a dummy cake to display... does anyone know where to purchase a dummy cake with that form already?  Or how do we do it?  Has anyone done this at their shop for display?

If you can't find a cake dummy, have you considered molding it from rice cereal? Might not be too difficult to do...

You could always carve a dummy into the shape you need.

Im not sure how long it will hold for... Id like to keep it for awhile... I am going to be spending a bit of $$$ trying to do an elaborate dress...

Several sizes here: http://www.buystoreshelving.com/jewelry_display_forms/

I don't know what they're made of, but you could cover in plastic wrap first.

Search for "mini dress form."