Rule of thumb is to use moss that grows on soil, as its unlikely to grow in your terrarium otherwise. Get it from your garden or public area.
Also, you can freeze the moss if you're not going to use it right away. Can be propagated by cuttings.
Likes moist, rich soil and well-ventilated, low-lit environments
Everything you need to know about making your own terrariums - type of plants, tips & display ideas!