I actually use regular sausage links(breakfast sausages), and cut off the casing or squeeze the meat out of the casing. It's just easier for me to measure that way. If I am making 12 scotch eggs, I use 12 sausage links like these:
http://www.magliosausage.com/images/...usageSmall.jpg
and take off the skin like this:
http://perso.orange.fr/philippe.torc...ges/dessus.JPG
However you can also buy sausage meet in bulk...it is sometimes packaged like ground beef.
http://www.cse.nd.edu/~pscherm1/foods/mvc-154x.jpg