As I have said before on here badger shaving brush knots are mainly made in China, they are available in the West from companies like The Golden Nib or, in quantities, directly from China. This means that to make your own custom brush all you have to do is turn your own handle, after which you can simply glue the knot you have bought into it. Many hobbyists are doing this.
But it also opens the door to small, artisan, businesses to provide low volume hand crafted brushes that can be unique works of art. Kimson in Vietnam is one such company, they specialise in making handles out of horn which they sell for $30 to $50. But the jewel in the crown is their black buffalo horn brush handle inlaid with abalone which you can see being made in the videos below. For this they charge $60, or $80 with a silvertip knot already glued in. Kimson handles are absolutely beautiful and have been well received by those lucky enough to own one.