We can usually get hold of deer sinew but getting anything from the abattoirs these days is one of those not happening sort of things.
If you only want it to bind a knife then the dog chews, made from the sinew along the spine of ruminant grazers, is fine. Just soak it and it becomes like wet chamois. Cut to size, bind, and it shrinks as it dries.
Best of luck with finding what you need.
cheers,
Toddy