Karaoke for Chloe Agnew's/Celtic Woman's Vivaldi's Rain?

Go online and try checking the online stores that sell karaoke CDs. Most sites have a "contact us" option. Use that and ask them if they have it (you might be looking for it in the wrong place) or if not, if they might be able to suggest where you could find it. It is possible that it is not available as a karaoke version. Then your only alternative might be to find someone who can play it and record it for you without any vocals.