After pricing 6" wheels at about $80, I decided to make my own.
I bought the thickest veg-tan tooling side I could find at Tandy leather for $35. It was about 3/16" thick. This was enough to make 4 six-ply wheels with a couple extra discs.
Using a 6" hole saw on my drill press, I cut as many discs as I could from the hide.
Caution: the hole saw frequently grabbed the leather and bound the spindle or burned the leather, filing my shop with smoke. They will be ragged, but I cleaned up the edges on the lathe after glue-up.
I made a simple caul to flatten and align the discs during gluing. Drill a 1/4" hole through two plywood scraps:
I glued the discs using yellow wood glue, three facing up and three down. Once the glue dried, I bored out the center hole and mounted it on an arbor.
I picked this 1/2"-bore arbor from HD because it was sturdy and inexpensive:
Final cost about $15 per wheel.