Dear Yohanl,

Your choice depends on the conditions you plan to have, key ones being light level and humidity. Here is one listing:
Hedwigia - sun
Entodon - both
Dicranum - shade
Ceratodon - sun
Climacium - shade
Thuidium - both
Polytrichum - both
Leucobryum - both
Hypnum - shade
If you don't have moss that you can remove from soil on your own property, ask at a local nursery that sells houseplants. They may sell you some cheap or even give you some that is volunteering in the greenhouse. Also you can go online to sites such as mountainmoss.com or mossacres.com to get "sampler trays" with several mosses that like sun or that like shade - or even a terrarium kit. For example, mountainmoss.com's kit description includes, "fluffy green mounds, fern-like carpets and tiny trees are represented in the moss species, " Dicranum, Hypnum, Thuidium,Climacium and Sphagnum". If you collect from your own property or (with permission) from someone else's, avoid mosses growing on rock or on tree trunks, as these have adapted to environments that are hard to provide in a terrarium. Whatever route you take, look up information online about the mosses. Those websites have some information and are worth looking at for only that reason. mossandstonegardens.com has a primer on growing moss. Even though it is focused on establishing an outdoor moss garden, the information there will apply in a terrarium. Go for it! Have fun!

The best moss I've ever had was what started growing naturally on my pumice stones I used for the fountain within my terrarium. After the constant recycling water soaked the stone the moss just started growing. There is also Selaginella kraussiana 'Brownii' which makes beautiful, if tempermental, emerald cushions.