Leather Sling
If you get a leather sling, make sure there is a suede or rubber backing.
I use a padded nylon sling on my Browning-hunting jacket, there is a piece of soft rubber sown into the top of each shoulder to prevent the sling from slipping. Nice touch, someone came up with that little idea, nothing drastic just finding a simple solution to a common problem. That way you can pick any kind of sling and not worry about it slipping off.
Wholesale Sports, and SIR must have Sako slings and there are some members on GunNutz that do leather work.
Also check out eBay as some of the other members suggested, you just have to keep checking to find what you want.
I like to use detachable sling swivels with locking devices on my slings.
If you find a good quality Sako sling let me know, as I need a couple for mine.
Albayo