The easiest would be a chain store like Micheals, I know they have an online store--but maybe look at Amazon first, they have better deals. 

Also, you could litterally just put scratch boards into google, and go into the shopping section, it lists it by prices, and sometimes you can find these sites in the dark corners of the internet that have ones that are silver scratchboards or multicolor.  Heh, I ain't going to tell you were to spend your money, but it's a good place to check out.