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?