Same as on Shared or Cloud hosting, you can have your domains protected by SSL on VPS as well! There are free and paid options available.
It's better to search for official documentation for the OS and template used on the server if an instruction wasn't provided by your hosting or SSL certificate provider
Free SSL options
First, check your panel's functionality - perhaps, free SSL is already available in it. For example:
For panels that don't have an SSL option and OS without panels, you can use Certbot.
Paid SSL options
You can also install any paid SSL certificate of your choice. If you prefer Comodo SSL, here are some step-by-step guides for you: