Fabric pots will prune your roots (especially when elevated to allow airflow around and under the pot) to keep them from circling and becoming root bound. If I'm not mistaken, it is due to the roots coming in contact with the air, and the roots pull themselves away.
I, as well as a friend, have purchased them through Amazon. For me, it was to buy a 6pack instead of individually at a store. I have seen them at Home Depot, Ace Hardware as well as the local nursery.