Most any grocery store should sell pork fat for sausage.
My local grocery store sells pork fat and it's near the sausage.
I've noticed that Amazon also sells Pork Fat that can be shipped to you within a day or two of ordering it.
Amazon sells just about everything and below is a link to the pork fat on Amazon.
https://www.amazon.com/Caul-Fat-Pork-Case-Frozen/dp/B003UMZ1JU
It's not cheap but it comes in 10 lbs.
However if you only need a little bit then your local grocery store should sell the pork fat.
Just look around for smaller grocery stores and I also think Dillon's Food Stores also sells pork fat as I believe I saw it in their store a few months ago.