Most likely sling found on a WW1 era P14 would have been a P1908 web sling, though a good many P1914 leather slings were also used in rear-echelon service.
The web sling is/was a better sling as it didn't rot like the leather one did in the trenches.
Sadly, you will probably have to settle for a ww2 era P37 web sling, which is basically identical to the P1908 web sling. WW1 dated P1908 slings are rare and expensive when found these days. I think all that changed between the two slings, if I recall, is that more materials were authorized for use on P37 slings, such as copper-washed steel fittings, steel rivets, etc. while on a P1908, only brass was used.