sorry about that, quickly read your post and thought you were after a backing board to mount your built-up, led letters onto, because you did not mention fixings. I just gave examples of the sheet material product because i doubt our suppliers would be much use to you as you are located in India.
so you mean the actual "fixings" to mount these letters?
im not sure if you are fabricating the letters yourself, but if not ide speak to the supplier of them for fixings.
you will be best with the sort of brass/metal screw in male/female locator fixings. these are normally soldered onto the metal built-ups.
http://www.fkmoore.co.uk/forms/brass_locator_nuts.asp