If you get from Petco, or mist anywhere, it's a good idea to quarantine or dip in bleach or potassium permanganate solution to avoid algae or other hitchhikers.
But, if possible, get in touch with other aquarists, and clubs, for plants or not. They can be great resources for plants,both free and cheap. And often far healthier and a better variety than you would get in your average pet store
I know Chicago, DC, NJ and CA all have good aquarium groups, there are likely others.. lits of other towns have general aquarium fish groups too. If you post your location I'm sure someone can help you find something close.