I used to collect plants from the backyard as a kid to put in terrariums.
Just look for plants with a tight, small growth pattern. You will find all
sorts from mosses to wild strawberries, ferns, and many little things that I
never found out the name of. If they get too big. Pull them out and
experiment with the next treasure.

If you want a sure thing, go to Kartuz.com. Michael sells miniature plants
including many rare mosses and miniature siningias. He is the one to go to
for a professional looking terrarium. He also has a large selection of rare
plants, vines, passiflora, and begonias. Fun website to dream over. Good
luck.