So I was looking at a server list and I noticed one of them had a <servername> core.phar plugin. What is this and how do you get one?
People make their own custom plugins just for their own servers, like i do. It is free to do, yet time consuming.
You can learn to make your own plugins, or ask some people who know how to to make for you. Whether it takes money depends on how well you know him, whether he is wiling to, etc. Plugin development itself has no cost other than electricity bills and time. It is just like asking to be done a favor.