I buy bunches of anachris, separate the stalks, clean/rinse them well and keep them in clean water in a five-gallon tank to feed to the turts as part of their diet. Anachris is an oxygenator and will root fairly quickly in the water given the right conditions. I've been told it roots even more quickly if its "planted" in the tank. (I've never tried this, though--the turts would just pull it up.)
Try to get aquatic plants at a garden shop that caters to people who have ponds. Don't know about where you are, but here, with fall approaching

, they often have sales to get rid of their aquatic plants (they just have to throw them out otherwise).
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-