There are actually three "issue" slings for the M14; the canvas type with the hook for the rear swivel and adjustable at the front end, the nylon version of the same style, and the leather type used by service shooting teams (it was never issued on service type weapons).
Any Army Surplus store worthy of the name should have the canvas version, and probably the nylon too. If you pay more than $3 for them ,you're getting ripped off.