try checking some of the sites that sell vivarium plants. sometimes you can find mosses or even spore mixes for sale.
I've also heard that you may be able to get moss spores at bonsai supply places, but those may be temperate vs. tropical...
You could just try harvesting some from outside - get a variety, especially mosses growing in dry areas, grind them up with buttermilk/beer, and paint the mix onto the wood.
I think you need to keep it damp/misted initially, but once they start to grow in, they should require less maintenance.