My favorite place to buy all things sausage online is The Sausage Maker: The Sausage Maker, Inc. - Sausage Making Equipment & Supplies
They are located in NJ or NY, but are able to ship wherever. I have always received decent/quick/dependable service and delivery from them. If you live near a Bass Pro Shop, I have been able to find fresh casings there in their cooking/smoking section--in little vac pacs with salt. Maybe you could find casings in some other, similar sportsman shop too? I also have purchased fresh from my local butcher, but he only had one size.
As far as casings go, you can get natural, collagen, and
synthetic. So you gotta be careful in which type of casing you order. Natural casings are, of course, intestinal lining. Collagen casings are technically still natural since they come from the animal collagen--but they are completely distinct from synthetic in that they are edible. Sythetic, however, tends to refer to a more plasticy casing that is not edible. Think of the casings that you find on a generic mexican chorizo--plasticy, rubbery, and not edible.
As for your question about whether collagen casings are any good, I really like collagen and get great results each time! The one thing to remember with collagen casings is that you cannot form links without tying them. With natural casings, you merely twist each link a few times to form an individual link. With collagen, you COULD twist them but they will unravel. When I first used the collagen, I was told you couldn't twist them. I figured I would buck the system and twist me out a few links. And it worked...at first. After I had "successfully" formed a few links, I turned my back for a second. I turned back around and they were all unraveled. So, I ate some crow, and remade links by tying cooking twine every few inches or so.
Generally, I found the collagen easier to work with and to store--a quick soak, and you are ready to stuff! And, generally, if you pack each link correctly, you will still get a nice "snap" when you bite into them even though they aren't fresh casings. Fresh doesn't necessarily add any flavor, and there is only a marginal difference in texture. But the tradeoff with natural casings is that you gotta work at unraveling the casings, washing them, packing it onto your stuffer, etc. prior to stuffing...not to mention trying to prevent shrinkage during cooking.
Overall, I prefer fresh casings...no particular reason. Maybe I just like the thought of working with a rustic medium? :lol:
But the reality is this: because of convenience in preparation, storage, and uniformity in my final product, I have made a ton more sausages from collagen than I ever have from fresh. Kinda like using USDA Choice meat over Prime meat for everyday use, I guess. :rolleyes: