I've made them from scrap polymer clay very sucessfully.
To use them, use a release such as a faint dusting of cornflour or a spray of water (I prefer cornstarch as I don't like wet clay and also I think it creates more of a barrier to protect the baked clay from the unbaked)
If you let the clay sit on the texture sheet too long, it will start to eat into it and make the baked clay gooey. So scrap clay molds aren't invincible, but they are pretty good and very economical to make
RTV silicone compounds are also good for creating molds etc, depending on what you are taking a texture of, you can use putty ones or build a mould with your texture at the bottom and use a more liquid pourable one