I recently tried some pre-tubed natural hog casings from The Sausage Maker. I was really impressed with the ease of use and the quality of those casings. I will use them again. I was caught short just before I was supposed to make some sausage for the guys at work and ended up calling a local butcher shop. That is where I got the pre-tubed casings.
You might call local "old school" butcher shops to see if they have tubed casings and will sell you some if you want to try them before ordering any quantity.