There are PE gun slings available, (I'd try Aber first) complete with buckles and other hardware. Try a search on Greatmodels, VLS or other online HS.
You can make your own from lead foil. I collect it from wine bottle hoods, (which now are mainly heavy-gauge aluminum foil, but work just as well). Use an X-Acto knife handle and roll it out flat on a smooth surface, (a piece of glass or ceramic tile works well). Using a metal straightedge and a sharp, #11 X-Acto, cut thin, (~.030") strips of foil. Take a look at a photo of a real rifle you are modeling to estimate the length needed and where to attach them. Buckles can be made by folding small bits of foil around the strap, glue with CA and trim excess to fit.