Things like lettuces and veggies you can get at the grocery store, or a farmer's market. I get an organic lettuce mix and share some with my turtles whenever i fix salad for me.
Here's the list of foods from the main site:
http://www.redearslider.com/plants.htmlBut the aquatic plants, yeah, only at a pet store. You could set up a little tank, if you have it, and stock it with aquatic plants so you have some extra on hand. It will need air flow and a source of food for the plants. But grocery store veggies will be fine, just wash them well in case they have pesticides or other chemicals on them.