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.)
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