I like using molds to model rocks. For many years I made and sold a line of over 20 molds. All were made from real rocks that looked good in miniature.
I still get requests from modelers who can't find the molds on my web site. I stopped making them over five years ago because they were very labor intensive and demand dwindled.
Nowadays when modelers ask about molds I send them to Sterling Models. They have a growing line of molds that are very realistic. Each mold is hand made using latex rubber and they're packaged to stay flat.
The larger molds are set in a cardboard "tray" for shipping. The tray is a handy holder when pouring plaster into the mold.
Check them out.