What other viable options do you really have (lamb, goat, tofu) in place of your current source?
To answer your question, rather than asking for discarded fat of questionable quality, I've been using fresh pork belly (as opposed to bacon or cured fatback) for my sausage or grinder needs. There's good quality meat in there, mixed in with good quality fat. It's not processed, it's not salted, and I've found it works nearly 1:1 as a substitute for the fat needs in a recipe.
I'd stick with pork belly. You're not making sausage because you're concerned about caloric intake There are a number of growing supporters to the fat/protein school of dietary thought, and I have to say that I lean much more favorably in that direction than I do the preachers promoting boneless/skinless chicken breasts.