my family buys them to make sausages. Sometimes big chain grocery stores carry them, just call and see, they might even be able to point you in the right direction if they don't carry them. Here's a tip, if you are using them for casings soak them in water with lemons in it, it makes them stronger for some reason.
most of those are plastic derived these days. Are you looking for sausage casings or menudo-cooking type intestines? Asian markets are always a good place to start, you can even get pig uterus if you have a good one. Is there a meat processing facility in your area (someone who processes deer/etc - legally)?