Still need more details, david. What are you trying to make casts of? Size, material? Will you need a vacuum pump, or a centrifuge?
If you want to make metal castings, then I assume you\'re interested in using silicone rubber to create the moulds? If you want a limited number of impressions, then greenstuff might be what you\'re wanting? Do you need high detail, or will low detail suffice, like what you\'d get from plaster-of-paris?
So, spill the beans, what are you trying to do?