Java ferns are known for being good low-light plants. Unfortunately the leaves are typically a bit big for your purpose. But there is a variety of java fern that has much smaller leaves. I've seen it sold as lace jave fern and bushy java fern. For a picture try:
http://www.aquariumfish.net/images_0...rn_rev_101.jpg
I'm sure some of our LFS that specialize in plants can get it for you. Here are two good stores to try
http://petshanty.com/
http://www.absolutelyfish.com/