I don't think the handles are commercially available, but @gssixgun posted about a bone or antler handle brush he made a while back.
It is really hard now since the supply chain SUCKs Antler is easier since we have Deer Elk and Moose here in Idaho and I can either beg for some or go hunt it myself These are the last two I made Reticulated Buffalo Horn Elk Antler with a polished Elk Bone Base
I don't think I ever posted these pics here I made this one years ago, I barely posted it on SRP and withing 2 weeks somebody had copies being produced in China or Pakistan, pretty much when I stopped making brushes other than to match Custom scales This is Turned down Elk Antler, pretty cool I thought, with a Turquoise Base, but not easy to do by hand
I hadn’t been in the lathe room in quite some time. Was happy to get in there to make a TSD raffle winner for @MoAllen : The wood is Gombe, which turned well. I liked the colours well enough that I made a similarly coloured handle in a different style for a friend as well.