For local, If you have a Bass Pro or Cabela's nearby they usually have plenty in stock (the LEM branded bags). Some Academy Sports have them, and so do some local hardware stores and chains like Ace Hardware. For the local places, it depends on demand so the more rural the hardware store the higher likely hood they have meat processing supplies. Otherwise, when I order online, I usually go with Butcher & Packer.
Like already suggested, ask your local butcher. I have one near me that sells pre-tubed casings in the single tube quantity. He goes through an order every week so I know his stock is fresh and I don't have to worry about buying a larger quantity and having the wife wonder what that weird looking stuff in the salt packed bag is in the back of fridge drawer. I just get a tube from him when I want to stuff a small to medium batch (and usually buy something else meatwise just to be fair since he helps me out). Local butchers generally will sell components like that. Worth asking.