Well, the best way is asking your dentist about the plaster he uses. Ask him if he can order some pounds for you and you pay him them, normally he will be glad to do it 
If you don't like that (or you hate dentist ) you can:
a) Use Plaster of Paris. It works great, I used it for my 6 first buildings, and they are there, well painted and great on the shelf.
b) Go to an artist supply store and buy some 'powder of stone', 'stone in powder', 'dehydrated stone' or something like that. or it's like plaster of paris, but when it's done it has a stone like touch and toughness. Problem: it's expensive.
c) Use a mixture of Plaster of Paris (90%) and 'stone in powder' (10%). It's cheap and more resistant than only plaster of Paris
But, in general, if you want only to build dungeons and use them in your games, without doing voyages with them, Plaster of Paris is enough. If you want sell them, carry them around (conventions, voyages) or something like that, go for dental plaster.
If you don't like that (or you hate dentist
) you can:a) Use Plaster of Paris. It works great, I used it for my 6 first buildings, and they are there, well painted and great on the shelf.
b) Go to an artist supply store and buy some 'powder of stone', 'stone in powder', 'dehydrated stone' or something like that. or it's like plaster of paris, but when it's done it has a stone like touch and toughness. Problem: it's expensive.
c) Use a mixture of Plaster of Paris (90%) and 'stone in powder' (10%). It's cheap and more resistant than only plaster of Paris
But, in general, if you want only to build dungeons and use them in your games, without doing voyages with them, Plaster of Paris is enough. If you want sell them, carry them around (conventions, voyages) or something like that, go for dental plaster.
