I would try Healthy Living or maybe the new Vermont Meat and Seafood in Williston (behind Rocky's Pizza) . Both make their own sausage so I am sure they have casings. If you go to the store in Williston, Eric and Dana (the owners) are great guys.
Do you mean getting them straight from a farm? I am not in VT, but I have found packages of casings at Stop and Shop and Shaw's. Not sure if that will really help you or not. Good luck!