are the graphics drivers for the khadas open source or proprietary
Proprietary. There are hacked closed source graphics drivers but so far for reliable operation these tie you to using the Android kernel 3.14
Are they available from the manufacturer or from third party.
Rolled into firmwares only.
oh ok when you say firmware you mean, the OS updates or is it a specific update that only works if we use. the stock OS that the board is distributed from khadas,
Weill I still get firmware if I install other distros on the board
Armbian uses the Android Kernel 3.14 which allows a version of the graphics stack to run with hardware acceleration. None of the other Linux distributions have hardware acceleration at the moment and there is a high probability that they never will (this is a licensing issue between ARM and Linux developers - ARM do not release opensource drivers so we are dependent on binary blobs which they only release for Android). There is a new Kernel with HW acceleration which is just about mature and is been rolled out to some of the Libreelec builds. this means it will probably be used in future Armbian releases.
So if you want to use hardware acceleration your choices are:
-use the stock Android ROM or an updated stock ROM
-wait for someone to port a version to Ubuntu (but be very patient)