I'm assuming you mean the item that you want to eventually be made of metal. I had mine 3D printed at my local library, or you can find a plastic and carve or the like, as long as you can melt it out, then you should be good.
Is it just plaster of paris, and not a mixture of something else with the plaster? I'm going to be casting with aluminum, which is about 1100 degrees when liquid, so will the plaster take that heat?
I've used that, but after the second burn, mine is at the failure point, so they do work, and are a great size for the Mini Foundry model, you'll need several.