If you want to buy a pothos or ivy from a nursery, you can then take a cutting from the plant and thoroughly rinse it with dechlorinated water. You could probably just throw it in your tank after that, although I would personally wait a little bit and make more cuttings after the original starts to grow. If you put the original in right away, keep a close eye on your frog for awhile, and absolutely make sure you thoroughly wash it.
You can clean and propagate wild moss if you really want to. Serpa Design has a wonderful video on his youtube channel. Just look him up.
Good Luck with your little guy's setup!![]()