Live plants are nice enrichment for them and do keep the humidity a bit higher. I've had some lucky bamboo in with mine since he was a baby and will be adding some new plants once his bigger enclosure arrives. You can always order plants from places like Josh's frogs. Amphibians by nature are going to be far more susceptible to pesticides than snakes so anything that is safe for them will be safe for your ball python. Definitely do pay attention to the soil you are using though - nothing with miracle gro, etc.