I grow most from seed. If you buy plants they most likely have had chemicals used on them. I just buy a bunch of edible plant seeds and throw them in the enclosure or plant in pots.
I grow a lot from seed. Other than that, either Lowe's or Home Depot. I just re-pot them when I get home and then wait a couple months before I use them for tortoise food.
That's going to be tough unless you find an organic nursery. I purchase wild bird seeds from the local feed store. They grow fast too. You'd have to let a plant sit outside of the pen before you put it inside.
I grow most of my own, when I do buy them I buy from a local nursery (local about 20 miles away) they don't spray so I feel safe using them, they cost more but I think it is worth it.
I buy from a local nursery, Lowes if they are vegtables grow them myself. I then transplant them into a small garden I have and let the grow for a short time. Normally if I put a small plant into my enclosure it either get eaten or trampled. So, I have to grow it out a little before moving it.