Gunbroker often has Yugoslavian or other countries who took the Kar98k in some form slings available.There are other ways of course. My reason for asking is I spent almost 4 solid years of my life everyday using a sling on a 10 meter air rifle and was a competitor at the state, regional and national level. I've always preferred to use a sling over a rest plus good practice for eventual match shooting with a couple rifles I have I would like to use in vintage rifle matches. (K98K would be more of a endurance test at a local match since there is one at a local range every month) As I stated I obviously don't use original slings for stability in shooting but I would like to find a good quality reproduction too use for that purpose.
That being said, any sling who has the same function (through the stock as the originals) will work. It doesn’t have to be a Kar98k sling or sling used on other military surplus. This means many other Mauser rifles whose sling used the same method of attachment would work. Lengths may vary but if you’re wrapping your arm with a sling etc then you might not notice.
It just has to function the same. You might realize the hype pricing around Kar98k or anything close to it will price you out for what you are looking for. Instead look for slings whose function can be used for the rifle and then check pricing.
If you go searching reproduction sling etc you’ll pay way more for the same gain.