I've never seen an OEM walnut offered in USA. Only the stained beech in sporter or thumbhole versions and blue or green laminates, or black synthetic thumbhole. Blued or "stainless" nickel plated actions and barrels.
that being said, I've seen a lot of custom stocks of all kinds of wood and laminates and configurations. Seems like one of the easiest platforms to find a custom stock.
As far as I've seen the only HWs with Walnut stocks are the HW35E springer and the HW100 PCP. Steve Corcoran from Woods and Water Outdoors probably makes them, he does excellent work and charges accordingly.