a good butcher will know what you are talking about. it is the fat around the kidney area and usually from beef so the elk stays pure. 
What does it mean to "stay pure"? Keeping the natural flavor and other properties as much as possible?
I was in a bind with my deer this year and ended up just using bacon. Worked great. Cubed up the meat, removed as much white tendon as I could, sliced up 1/2 frozed bacon in small bits, hand mixed in white tub, added flavors\seasoning, chilled in freezer until 1/2 frozen, grinded 2 times. Made awesome burgers and sausage.
For sausage, I just used some pre-mixed flavoring from Cabelas that I didn't end up caring much for. Will try something else next time. For burgers, I just mixed in some worchestire sauce, garlic salt, and black pepper.
Got a meat grider attachment for my wifes heavy duty kitchenaid mixer and spent a few hours griding away (not 2 hours straight grinding - other stuff mixed in...).
Turned out awesome.
Don't have a vacuum sealer yet. So just put the burger and sausage in freezer zip lock bags. Other meat was wrapped in butcher paper. Won't last long in my family of 10, so not worried about freezer burn or anything like that.