Hi Shivani,
You can purchase markers that have 'classic' ink which is waterbased, 'craft' ink which is waterbased but different formula, or a pigment ink, or solvent based - think Sharpies permanent ink. If you are looking to add colour to hand drawn images, I would suggest that you buy a few good quality artist type drawing markers in basic colours ie black, brown, grey, dk blue. Use these pens/markers to create your image and I would suggest that they be a pigment or solvent ink so that you could then use other sources to colour in your images. Once you are ready to add colour to your images use things like ink in a bottle to mix on an artisit plate for colours, peerless watercolour pages ( paint on paper just touch with wet brush), markers, pencil crayons, watercolour crayons etc.....
If you go to google or another search engine you should find all sorts of suppliers for supplies.
May I ask are you in North America? You could have trouble finding shipping if not....
Hope that helps and good luck