Chaeto seems to be the defacto..
liveaquaria.com sells it.
That said, I have a variety of things (got a 6 pack from ebay, some from the lfs
and some from liveaquaria).
The reason you have to be careful of what you put in is because some of these (like grape calurpa for instace - which seems to be the most abundant I can find locally) will spread fairly rapidly and even go 'sexual' - releasing spores into the water column that could potantially take over the tank (and kill stuff).
Proper care and pruning keeps this in check, but you have to spend 'alot' of time and effort obviously (I can't stay away from my tank, so it's no biggy for me) as well as prune properly so as to not damage and kill the plants, which release all the toxins they helped remove in the first place.