I remember looking for years for my first carbine sling. Seems like they show up every now & then these days. E-bay is a good place to watch. I have run across a couple that are longer than the prescribed 40 inches. The main criteria I've found is the 1 1/8" width, a square buckle (rifle slings are a narrow rectangle), only one position for the buckle (rifle slings have two positions) and usually two keepers. These can be found in both leather and rubberized canvas. here's a couple of photos of one of mine. Good hunting!
"Now remember, when things look bad and it looks like you're not gonna make it, then you gotta get mean. I mean plumb, mad-dog mean. 'Cause if you lose your head and you give up then you neither live nor win. That's just the way it is"