I bought a 1907 pattern sling, $25 shipped on Amazon from International Military Antiques. I gave it a good rub with Lexol leather conditioner, but it's still fairly stiff.
It adjusts from carry-loose to rack-tight nicely, but the top frog keeps falling out while I'm tightening the keepers on my arm.
Is that something that happens less as a sling breaks-in, or did I just go too cheap?