Levy's makes some superb slings. They are made in Canada. Finest I have found for staying on your shoulder.
Vero Vellini makes some nice slings, but show a lot of wear after a few seasons.
For leather, the old Bianchi Cobras and the sling provided by Remington with their 700's work well and last forever.

Imagine your grave on a windy winter night. You've been dead for 70 years.
It's been 50 since a visitor last paused at your tombstone.....
Now explain why you're in a pissy mood today.