If you want just a small amount, sometimes you can get some from local sign shops (ask for scraps - they sometimes give it to you). If not you can at least buy it from them. If you are looking at getting a quantity there are different places on line - I like
www.hhsignsupply.com they have good prices and great service. If you buy a 10 yard piece of permanent vinyl it is about 14.00.
The vinyl I like is the Oracal - 651 (permanent) or Oracal - 631 (removable)