There is a specialist shop in Sanur on the Jalan Danau Buyan in Sanur.
From Sanur cross the bypass by MC Donalds, to jalan Danau Buyan after 500 meter on the left side there is a Cirkle K, next to is the shop.
Never been there, i don't paint, but see many paintbrushes and accesoires
Good Luck, Hugo