I agree with using something like a woolen Army blanket, or wool yardage if you don't mind paying more for it.
I just recently finished a quilt for a cousin that lives in Massachusetts. For the batting, I used three yards of thrifted vintage wool flannel that came from a mill in a town where his father was born. There wasn't quite enough so I had to buy a couple more yards from an online source (NOT cheap!), but the quilt turned out lovely and it's very heavy and warm. Good for those cold Mass. winters!