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...
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