I got a GI sling for around $20 at Action Military Surplus on Auburn. I went there and asked for one and they showed me some other slings. I wandered around the store and found them myself. The part that attaches to the swivel was very tight though so try to find one that doesn't look mashed.
If the CMP still has them that is the cheapest way to go. Not sure if there is still enough time to get one from them.
sling link