If you're going to find a bait donor tail (which I agree would be the easiest and best match, as first set Rainbows are quite common and horrible baity Starshines and such are not that hard to find either) it might be worth taking note of how vivid the pink is in your Moonstone's mane. I've noticed that there are a couple of shades of pink that are used in the first set Rainbow Ponies (or it might be age deterioration, but it isn't fading) and that might affect the colour match in the tail.