As I'm not a big sausage fan I'm not quite sure where you can buy skins. But I do know this:
a. You would probably need a special machine. Try Grobstein at http://www.grub.co.il/ to see whether they have such machine and/or sausage skin. Another option might be any sausegier.
b. Making sausage is as scientific as baking. You must use an exact porportion between meat and fat. Try consulting the people of the ort food forum on: http://demo.ort.org.il/ortforums/scripts/forum.asp?pc=393235558
Doron.
I have just read a brilliant article in "Zman Modiin" one of Modiins local newspapers about how to make sausages, also the butcher "Lomito"in Mercaz Sharvat sells sausage skins.
Hooray! Lets get the barbecues out.