It's part number 50901346. You might have more luck finding a tailgate with spoiler in your colour from a breakers, much cheaper and easier than fitting and painting a new one IMO.
Oh right, yeah you're right it's part of the Linea kit but as far as I know it's a whole different spoiler. Your chances are pretty slim of finding it, I think SJB1985 has one but I daresay it didn't come easily or cheap!