I want to write stage adaptations. How do I buy rights so I can do this legally.?

It's highly unlikely that the writers would give you any rights at all. The chances are that if they wanted their work made into stage performances they'd either do it themselves or employ a well-known and very established professional to do it.

But if you want to try, all you can do is write to them, give details of your experience, qualifications and previous work. Just don't expect a reply, let alone their permission, and don't even think of going ahead without their written permission and a full contract!

Write to the publishers and ask permission. Mention what production agency will be producing these plays if you are adapting them for a particular group. Better you should look for material whose copy right has expired.