Hi,
In my terrariums I use:
dwarf mongo grass, looks like little clumps of grass, growing about 2-3 inches tall
pilea, baby tears, creeps across the ground, but will also grow up the sides of the tank, sending roots down anywhere and every where, both I got from esty.com "terrarium house". I also like reserrection fern, it grows around 4 inches tall, looking very much like a miniature fern, it will grow on the ground, but also makes a great background cover. The best price I have found for the size (about a sqaure foot) is again on esty.com "mr. nature". Both of these sellers I have dealt and can recommend based on my expereince.
There is also, any of the selaginella family, specifically, I have unumata or peacock spike moss, available at either Josh's frogs or also on etsy.com. Again creeps across the floor. I did get mine from Joshs frog, but found it cheaper on etsy.
I should add that the terrariums in which all of the above grow well for me have approximately 80% humidity.
I hope this helps.
