All you need is java moss. If you are planning on keping dart frogs... you should be able to have java moss work for you with plenty of moisture and decent lighting. Java moss is an aquatic plant that can be found at many fish stores. If kept quite moist for the first little bit... it should form a nice carpet with proper lighitn. I wouldn't go anything less than two fluorescent tubes... but that would be an absolute minnimum for any sort of growth.
Since you live in the US.. Black Jungle may have some.... as well as some other types of terrarium moss. Here is a link to their site:
www.blackjungle.com
http://www.blackjungle.com/Merchant2...tegory_Code=FM
Go to the bottom of the page on the second link to find starter clumps of java moss.
Good luck