How do I get a light source do i buy it somewhere or I have to find it or it's part of the main quest? Please help me out.
earliest/easiest option is to craft a torch at home. IIRC you need to build the workshop first, for which you need to buy five planks from Logan at the sawmill in Bareshade. (any future planks you can then craft yourself.)
alternatively, Othra the lizard priestess gives you a glowing amulet. IIRC you need to have picked up some glowshroom to unlock that dialog option.