how can I buy a QT 7 registraion key?

I would start with calling the Apple online store to see if it is still available.

1-800-MY-APPLE (1-800-692-7753)

Late 2012 Mac mini, OS X El Capitan 10.11.5 beta 3;  Watch, 38 mm silver AL, Watch OS 2.1; iPad Air 2 & iPhone 6+, iOS 9.3.2 beta 3;  Apple Airport Express