They can usually be found on eBay without too much effort like
this one.
Buckled green canvas with a riveted end with one hook and a leather keeper on one end is typically what you're looking for. The hook end on the sling goes through the bottom swivel under the stock then through the keeper and buckle to control the loop size and finally the hook clips to the front gas block. They are kind of a pain to deal with if you disassemble the rifle a ton and lots of guys simply don't put them on because the metal clip has a tendency to scratch the finish up at the gas block.
There are variations of slings out there too, blue ones from the Navy, I believe some all leather, slings for other guns like PPSh and SVT-40, etc. I'm not sure that we have a good thread on Soviet slings, but the AK/SKS slings are relatively easy to ID. They also have dates on them and some guys try to match the sling date with the rifle date.
Hope you find a good one!