https://www.spigotmc.org/resources/official-mcmmo-original-author-returns.64348/
there is the link to the original download page
I believe the plugin is free, and the payment is an optional donation, although I cannot find the free version.
Edit: found the free version
it's here https://github.com/mcMMO-Dev/mcMMO
although it has to be "compiled". I'm not sure what that means, there are instructions at the bottom of the page.