AlexPalmer
You cannot use any processed man-made products such as rubber as a substrate, as this will leak chemicals all over. Cheap and easy substrates that don't look very good, but that are easy to clean are newspapers and paper towels. Better looking, moisture-retaining, hard to clean substrates include soils and mosses (which must be bought from a pet store). Bark is available, but it's preferable to avoid this because any small pieces are indigestible and will kill the animal. Sand is just bad in general for tree frogs.

L Dennis
I wouldn't recommend rubber. Moss would be good, but make sure if you pick the moss yourself you boil it to kill bacteria. Some other good substances would be: Bark (Be careful what type of tree it's from as some wood is poisonous) Aquarium Gravel or small rocks Soil (Without any added chemicals) Plants (Non-poisonous, of course) Shallow water dish (Some white's treefrogs seem to like sleeping in water) White's Tree Frogs seem to prefer bedding areas where they can hide and be in the shadows all day.