I buy pretty much all my seasonings and casings from butcher-packer.com. They are located in Michigan, I believe, and you can order right online. Very fast service, too.
If we are making snack sticks, I get those casings (19mm collagen) from sausagemaker.com. It is the only place I have seen that size.
The casings we use depend on what we are making. For bratwurst type sausage, we use natural hog casings, breakfast sausage just gets packed in meat bags (no casing), and summer sausage/ bologna type stuff, we use synthetic fibrous casings. Casing size depends on the intended use of the sausage.
I am sure there are some really good pre-mixed seasonings out there, but all our stuff is from recipes we have collected here and there. We are still pretty new to this, having only made sausage for 3 or 4 years now, but it is great fun and we havent "laid an egg" yet.
For anyone who wants to start making their own sausage, they should first buy a copy of "Great Sausage Recipes" by Rytek Kutas. I consider this the bible of sausage making with loads of information and recipes. It is available from sausagemaker.com (Rytek started the company, I think), Amazon, and other places. Well worth the money. We havent found a bad recipe in it yet, either.
Making sausage is fun, easy, and you can fine tune your recipes to taste. Cant beat it IMO.
BTW, the sausages in the picture that are in the smoker, are about 14-15" long, and 2" in diameter. If you look, you can see I had to make a few pieces to raise the top of the links higher (risers the smoke sticks are resting on), and I replaced the the water pan with a formed aluminum piece to give me more room at the bottom. Otherwise, I couldnt have smoked these links. That is my main reason for wanting the 6 rack Bradley. More length.
BP