I've been having trouble finding good quality fat back at a decent price recently, so based on some advice from a very good sausage maker, I tried pork jowl (cheek) as the fat source. The bellies I've been getting lately are also quite lean. I was able to get my butcher to order 4 of them and they looked to be over 50 % fat (hard to see this in my photo). The fat is lower melting than backfat (I didn't check this), but they made up very good sausage without an excessive loss of fat while cooking. The jowls also were half the price of back fat and available with some advance ordering.
If anyone else is in this position, I can suggest that pork jowl may be a good option.