If by "plaster coat" you mean Sculpt or Coat (which Drew Allison uses extensively I believe) it can be purchased from:
Sculptural Arts Coating, Inc.
501 Guilford Ave. Greensboro, NC 27401
1-800-743-0379
They also have a website -
http://www.sculpturalarts.com (click on the "puppet" page)
Other alternatives would be to use white tool tip or plasti-dip, which is carried by hardware stores or rubber latex which can be purchased from almost any art supply store. I'd recommend going with Sculpt or Coat or tool tip rather than latex, if only because latex tends to add a lot of weight to a puppet and it's really, really stinky.
With any of these products it is usually a good idea to do several thin coats as opposed to one thick one to get the best results. Some of these are toxic as well so you should be sure to observe the proper safety proceedures.