nurseries, garden centers, home improvement stores. I would recommend keeping them in posts for the first year so that the tortoise can't eat them. You will want any pesticides that may have been applied to exit the the plant before the possibility of feeding.
I take cuttings from pothos plants that I buy at Home Depot and start them in water; once they've taken root, I plant them in mulch or orchid bark and after a few months I transfer them into my enclosures.