The moment you sit and think what is the origin of thought the mind goes blank. That indicates from where thoughts are coming.
You are the source from where thoughts, ideas and emotions come – that inner space.
It is like someone asking, 'Where do the clouds come from?'
Clouds are just hovering in the sky. In the same way, in our inner sky, there are three akashas – one is the outer space, the second is the inner space where thoughts and emotion come, and there is the third space which is a witness. Where there is nothing, just bliss.