Depending on how a site is hosted and where, there are various ways of adding an SSL certificate. In some cases, if there’s an ecommerce element on the site, it will be a requirement to have a certificate. Major hosting providers often offer hosting packages including SSL certificates.
It may also be possible to transfer an existing SSL from other hosts (exporting it from the original server and importing it on the new server). It will be necessary to follow the specific instructions on the webhoster’s site. Note that some Certification Authorities require you to purchase a server license for each server that will host the certificate.