I found this online. Not exactly what you're looking for, but I hope it helps:
"Gypsum Molding Materials: If you cannot find Ultra-Cal 30 Stone outside USA, I hear that a brand called Crystacal R (in the UK) may be substituted. Other option: Internationally-worldwide, every major city has Dental Supply companies. You can use Dental Stone (not plaster) which have different names from dental suppliers; the Stone with the least expansion of about .07% or less. Also, do not get Dental Stone fortified with resin. I do not know how that re-acts to baking & foam rubber latex. Just use the harder density, dental gypsum stones."
I am told that Crystacal R is so so, you might be able to get that. Honestly though, for a really hard solid mold, I would go with dental stone. It is available in virtually every country in the world, just look for medical materials/dental materials suppliers. It tends to set a bit faster than Ultracal, but there are often choices in hardness and set time. In my opinion, the harder the better. If you plan to dwell latex in it though, it will take a really long time. It gets amazing detail though.