I have some wool trousers that need a hem. And I have some extra bison hide. Looking for some tips. thx.

I have done this with some moose hide to one of my pairs. I used a glovers needle and artificial sinew and hand stitched it. That said depending on the thickness of your leather you will want to use an awl to pre pierce the leather, alternatively you could use a stitching fork to poke holes and set your stitch length. If you sing the stitching fork I would use the artificial sinew for thread and saddle stitch with two needles for the strongest seam. Also I would be wrapping the leather into a U shape and sewing through both layers of leather with your hemmed pant layer in between.

I have not done quite what you are doing, but have sewn with moose hide as well as beaver and coyote fur. I second using a glover which has a triangular point vs a rounded point on a standard needle. I've sometimes just seen them called leather needles too. A few times when the leather was tougher and I could not push through with fingers I would use a needle nose pliers to push and pull through. On one project I just used a standard needle on my sewing machine and it worked through 2 layers of moose hide. I have since learned that they make glovers for sewing machines, which I'll use next time.

For thread I've always just used a thicker polyester thread. Gutterman Mara 70, 100, or thicker are popular with DIYers.