Turner or get one made from a leathermaker (perferably one that specializes in gunleather).
Short of that, the cotton or nylon slings that came on Garands (I think) will suffice in place of cheap leather. We couldn't convince the Bde. that we needed 40 of these Turner slings per class for SDM training at our school back in '03 (along with custom rifles for the SDMs). What they did do is give us the keys to the warehouse where all the old stuff is stored, and we grabbed a bunch of these slings. They can be modified into a suitable NM shooting sling in a pinch.
So the very first army SDM rifles were our regular 14.5" Colt M4 KAC rail uppers, sometimes mated to older A2 lowers. They used an ACOG (but started training with the carry handle) Harris bipod, two KAC bipod mounts (one on the bottom for the bipod, one on the side for the sling) and that modified Garand sling. Some got modified at the SF armory and were upgraded with the SOCOM barrels. All had the usual 12lb. burst triggers.
Later they got the old M14's re-issued (you have no idea how much begging and pleading and paperwork and time went into getting them to open the racks and re-issue these). Now some have better custom stuff, some take their own personal uppers, and some soldiers even get their CO's signature to take their own weapons. My buddy carried his own M21 with a Turner sling to Iraq, and his own pistol. I don't think he ever drew a military weapon the whole time in country.
Our shooters had to qualify on a NM shooting course to 600m with irons using this setup in order to move onto the ACOG. Having shot this course myself, I can say those slings sure did make a difference.
Bottom line though, is that if we couldn't get good leather slings, then what was the point? Better to just modify these things, and they can be had for almost free at surplus stores. I think they work better than cheap leather, and we were in fact surprised at just how well they ended up working out.
Funny thing is, that many graduates ended up keeping those slings, preferring them to anything else. I still have a couple. For competition, I'd use the Turner. For fighting, I'd use the Garand sling set up like a Turner, it is easier to use and it works. It works pretty well on the AR, better than you might think, and considering the cost, worth a try whether or not you need the Turner.
