The kerr slings are still made today, and have been made since WW1. They came standard issue for the M1917 rifle. E-bay is full of Kerr slings.
See here for how to mount and use one:
http://www.armscollectors.com/17sling/
Also, Whatpriceglory sells them, google their name and "kerr". They are $25 each and are well made reproductions of the WW1 variant. GREAT sling for actual use. Get the M1917 length. They make shorter versions for the M1903 and the Thompson, it's the M1917 version you want.
This is the one you use?
http://www.ebay.com/itm/Long-WWI-Ke...894843?hash=item35f4d56e7b:g:CsAAAOxyY3ZRwy5X

