Check online. I am away from my regular everyday PC, where I know I have several sausage supply websites bookmarked. I will check there in a few days when I am back home. In the meantime I can assure you that there are a good number of choices for such things.
Alternatively, you might check with a local sausage maker, such as Karl's Sausage Kitchen. If they are not willing or able to sell you what you need, they may be able to point you to a local supplier or order some on your behalf.
Karl's Sausage Kitchen
142 Broadway, Saugus, MA 01906
For small quantities you are probably best ordering online like at butcherpacker.com which has several options. There is a casing importer in Medford on Main Street which will sell cash and carry, I believe all their natural casings are in solution not salt packed (but can be frozen after opening) and are usually packaged for 100lbs of meat. They may have a minimum order too -- I have always spent over $50 and you have to call first. (I had their number in my phone contacts, but am not finding it if you are really interested I can dig it up or Aromatherapy can probably give you at least the name.)