The guy who's making my Hirst stuff is using herculite 2 plaster. Its a pretty solid material, and less prone to bubbling than regular plaster according to him (though of all the stuff I've bought off of him I've not found any problems with the casts).
What are your molds shaped like? Are you casting these up as solid pieces, or as say two "L" shaped identical parts (each containing a top/bottom piece, a side, and an end)? For some reason those companies that use resin for their shipping containers seem to go for a single piece, which wastes material and sends the price up. =/