Ultracal can be found at specialty cement suppliers. Try doing a search for places close that carry US Gypsum products. for a mold like this you have to do it in 2 pieces and plaster of paris expands way to much to use and it's far too weak. If you decide to start over look up some vids on making a 2 piece ultracal mold. you have to do it in several layers and the first is always a thin liquid that's brushed on with a soft natural brush to avoid air pockets. I usually do 2 thin layers then a layer with burlap reinforcement and a finishing layer over that. Also making a proper slip latex mask is best done by pouring the entire mold full and letting it sit for a specific time depending on how thick you want and then pouring it back out letting it drain on something over the bucket. I've done many this way.