Dwarf hairgrass (Eleocharis parvula) might be an option for you. You'll want to break it up a bit into smaller clumps. Its not quite a moss, but it can grow and spread across the base of your tank and will only grow a few inches high, without trimming. If you want it lower, you can always trim it. ;-) Works best in a fine substrate.
For a big bushy plant that is very easy to grow and readily available anywhere, amazon swords are a nice busy plant. There are many different varieties, from broad leaves to narrow, and varying heights. They are also sold in various sizes of maturity at most pet shops. Works in nearly any substrate.
Anubias sp. are another variety of plant that can be quite attractive. These are also very easy to grow, but need to be tied on to some driftwood or rock for best results. They don't like being buried in the substrate.