Its called pillow moss and it comes dry with 3 or 4 nice size pieces in a container. I get my Materials from a local reptile store. The moss will grow under the right conditions.
Beautiful tank! Leucs are a great choice for such a large tank because they're one of the only Dendrobatid species that do well in groups with more than one female. Can't wait to see a colony in there!
i'll have to post some pics of my vivariums i custom decorated inside. They turned out pretty amazing... i have a sexed pair of azeureus coming in, some leucamelas, and some auratus. I'm quite excited.