I buy mine at a LQS or online. Lots of places online carry it: Amazon carries it, and so does Create4Less, Nancy's Notions, Connecting Threads etc.... even Joann's online carries it. Just do a search and find the best price.
As far as natural, in the past I did use vinegar and salt but ran into trouble a few times where it didn't work and ruined the gift (long before Color Catchers)..... Nowadays most people use Color Catchers. I've also used OxyClean to remove color after it bled and it has worked). You can Color Catchers and OxyClean in most stores.