Back in my day the local telephone company used waxed lacing cable for that sort of thing[1]. These days it seems that polypropylene string is popular (search on "conduit pull string").

You basically want something that is slippery and will tend to not get stuck. I have used Dacron fishing line, but that is mostly because I had a bunch of it laying around.

[1] https://en.wikipedia.org/wiki/Cable_lacing