Not sure where to buy online, Ive thought about it though since its kind of hit or miss at my LFS. Let me know where you end up.
I always think
Java Moss is really cool, and is very easy to grow. just get a piece of fishing line and tie it to where ever you want it to grow until it takes hold of the surface, or drop a few peices of substrate in it and it wont move around on the bottom.
Pygmy chain sword is another easy to grow plant you could use as a slightly more foreground plant in your tank, it grown to about 3-4 inches tall, so it willl add some variety to the front/ lower levels. It is super easy to grow also. It does reproduce and spread fairly quickly, so be prepared to trim a runner or two from time to time.
You could also add a few floating plants as well,
Water Sprite as well as
Amazon Frogbit will grow just fine in your tank. Frogbit reproduces quickly which i think is really neat.
Water Sprite will show more of the plant submerged in the water, but I think the looks of the dangling roots of the frogbit is really cool.
I'd shy away from the stemed plants, they are a pain in the neck to keep them trimmed back.
Hope some of these sugestions inspire some new ideas for your tank.
Sean