I typically just buy it from Home Depot and rinse the dirt off the roots. I also rinse the plant/leaves really well too. Then I just put the roots in the water of my aquarium.
If you’re nervous about chemicals on the roots then you could let the roots soak in a bucket of old aquarium water. Just change the water in the bucket when you do a water change on your aquarium. I think you would be good to go after a few weeks of water changes (assuming you want to add the plant to your aquarium).