I buy my brushes from Coastal Scents. They have a 40% or 50% sale a few times each year so I stock up then.
Since the brush heads are odd sizes, I drill the hole for the brush head with a forstner bit. I then adjust the hole with a small bent hollowing tool according to the brush head. I shape the handle, flip it around and mount it in a chuck with some 1" pin jaws in expansion mode and then finish off the bottom. I use a beeswax/mineral oil finish that is easily reapplied buy the buyer.
Here are a few I've made...