Fatback substitute?

Virtually all of the recipes in the Rulhman Charcuterie book suggest using fatback for the fat component of any fresh sausage. What could I use if I don't want to use fatback? I have no problem with fatback, but am planning on making some sausage for a friend who keeps kosher. Would duck fat be a good choice? Is duck fat available for purchase in non-rendered form? I will be making duck sausage, beef sausage and, possibly, elk sausage.

Googling "kosher sausage recipes" came up with this Chowhound thread on kosher sausage casings. They suggested veal fat, or some kind of fat from called fertiloff (sic.) At any rate, Roumanian Kosher might be a good place to ask.

Romanian Kosher Sausage Co
7200 N Clark St
(between Chase Ave & Touhy Ave)
Chicago, IL 60626

(773) 761-4141

if your friend truly keeps kosher he/she might also be concerned in whether the meat you are using is kosher as well-