Can anyone tell me where I can buy rainbow edible paper

Are you looking for an edible image or something more like wafer paper?

 assuming you want wafer paper.  Fancy Flours has a large section, not sure if they have rainbow design.  Cake Art will print custom sheets if you have a pic to upload.  

Of course it depends on where  you are located.  My local cake supply shop carries small amounts of wafer paper.  You can buy a single sheet or 100.   Have your checking with a shop near you?