You can create, install, and use your own certificate (called a self-signed certificate) but it won't be trusted by anyone's browsers. In fact ugly popups or a warning page will be sure to deter visitors from proceeding onto your site.
Self signed ssl certificates are useful for testing and development purposes, but I would advise against using those certificates in production environments for the reasons listed above. That's why you buy from a CA like ssl.com or other trusted authority. They are trusted by all browsers, and their issued ssl certificates will not result in warnings from browsers.
Self signed ssl certificates are useful for testing and development purposes, but I would advise against using those certificates in production environments for the reasons listed above. That's why you buy from a CA like ssl.com or other trusted authority. They are trusted by all browsers, and their issued ssl certificates will not result in warnings from browsers.