The source of inspiration can be any of the things:deep emotional experiences - say, romantic love or spiritual contemplation.I think such rare moments come only when you have total concentration. You are consumed in and by the music. I guess you could say that it is akin to contemplation. In order to reach this desirable state of mind you have to rise above the environment you're in at that particular time - a bad piano, glaring stage lights, or the attitude of the audience. Sometimes the inspiration of the other musicians you're playing with helps you reach this stage.