I honestly just use the Deweid casings from Amazon but I've heard great things about these guys:
http://www.makincasing.com/
LEM is selling hog casing that's already pre-tubed as well. It keeps forever if it's heavily salted in the fridge, I just worry about using it all before it goes bad. I just make a 5 or 6 pound batch 3 or 4 times a year.