How to get Open Brush
Open Brush is completely free on all platforms. (Tilt Brush is still available and is still being charged for. This is still the only way to get the app on Playstation VR)
Only the main official release is available on most of the links below. If you want the experimental build then try Itch.io - or read about the other alternate versions that are available
There is no VR headset currently supported on the Mac but Open Brush does have some limited support for working as a normal desktop app.