Every one of my historic and replica BP rifles has correct pattern slings made by Laurence White here in England. His products are made to the exact same patterns as the originals held in the royal Armouries and MoD Pattern Room. Post#25 refers.
Here is one of Laurence's fine slings on my original Pritchett rifle, one of the last he made before he went bust and took up water-colour paining for HM Queen Victoria. Apparently he was quite good.
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Since I don't know where you are, please be advised that there is another craftsman whose products are at the same level based in BC. Rob Enfield's guest shooter on a recent programme he made about the last Fenian 'invasion' of Canada had his guest fully-equipped with totally correct leatherwork.
You might get in touch with Rob to get his contact details. If you do, tell him I sent you. The gentleman's name is FNU Branko, BTW.