To purchase a SSL certificate, you should go to the SSL Certificate Provider (certificate authority). Each SSL Certificate Provider has different products, prices and levels of customer satisfaction, which is nearly the same logic like purchasing web hosting. To check the best offers and products you can try SSLShopper by clicking on the following link.
https://www.sslshopper.com/certificate-authority-reviews.html.
It has reviews and experience from other clients that have purchased before a SSL Certificate.
The purchasing process then takes place as depicted in the following illustration.