Just google it mate. I had a quick look, pleas click on link or copy and paste it in to your browser. You will be able to buy a gun sling online. Be careful though as I notice the specs are based on barrel width, with the HW77 it's the underleaver part that you attach it to, not sure if this is the same diametre as the barrel? If you can't get a swivel small enough you could just buy a bigger one and just put a centimetre of rubber (bike tyre inner tube or something) wrapped round the underleaver as a filler.
I had an HW77 for years, great gun. If you are not using Elay Wasp pellets a