While I agree Mutton is about the best.
In actuallity you can use any fat if need be.
Save the fat from your deer, bear elk, pronghorn
render it down.
Save the raw fat from your steak, or ask the butcher to save some.
You can even save bacon grease and render it down.
Yep a PITA, but as a sport and a hobby.....
If need be buy a 1lb block of hog lard at the grocery store.