We needed a head model for a costume helmet fitting, so we made a head mold from Alginate and cast it out of the plaster Hydrocal.
Mold: Alja-Safe from Smooth-On
3-Lbs bag should be more than enough for a bust
Mother Mold: Plaster Bandages
Best deal is from Pacon Craft
It’s $50 on Amazon, but way more than you need