Where can I find a digital copy of the script for Noël Coward's Blithe Spirit?
As the other Educator has already stated, Noël Coward's Blithe Spirit is still under copyright and not in the public domain; thus, it will be unlikely (and, in fact, illegal!) for you to find a free copy of it floating around on the Internet.
That being said, Samuel French has a free sample of the play available as a PDF (which I have provided a link to below). You can purchase a paper copy directly through their website.
You might also consider checking with your local library. If you are really set on having a digital edition, you might inquire about your library's digital loans policy; many libraries now allow you to check out an eBook onto your Kindle or other reading device.
If you are in a great rush, Amazon has Coward's collected plays (including Blithe Spirit!) for sale as a Kindle Edition for only $9.99. Personally, I think that's a great deal and would be your best option! I have included the link to this page below. Happy reading!
The play Blithe Spirit by Noel Coward is still under copyright, and thus free digital copies are not available legally. You can buy print copies of the play from Samuel French, the copyright holder, a company that is also in charge of licensing productions. If you want to make a digital copy for your own use, you can buy a print copy from Samuel French or a internet or bricks and mortar bookshop, or obtain one from the library, and then scan the play into a .pdf file. To make a searchable copy, you can attempt to convert the .pdf version to a document file using OCR technology.