I'm using buff colored dental stone for the casting material which is easy to work with and is very durable once dry. I bought a 25lb box on Amazon for $35.00 bucks and its was delivered to the house for free the next day. Amazon Prime is an amazing thing.
The molds themselves are not cheap at $34.00 each but I've both been read and told they are very durable and you really open up a range of creative opportunities for terrain.
It takes a little less than 30 minutes to prep the mold, pour the plaster and then have it set so it's best do work with several molds at once.
It's funny in a world of 3D printers sometimes the old fashioned ways still are the best.