Anyone who programs can make new plugins. We expect community minded organizations to create and publish plugins as open source. Here is how to find these folks.
Look for Their Sites
If you find a page with a plugin you don't have you will know because it renders as "Can't find plugin 'crazy'" or something maybe more helpful.
We don't expect wiki to find the plugin for this crazy item. For a paragraph of unknown type wiki shows the text field, if any, and offers whater help it can.
You can trace items with missing plugins back to their source. Look for forks in the page's journal. These sites have plugins too. Visit them to see if the page renders better there. (Click the flag on a remote page to see how it is rendered from that sever.)
Once you find a site that has the plugin you can look to its Welcome Visitors page for contact information. Read further or write to them for details about their plugins.
Once you find a site you have probably also found its documentation. Try double clicking the rendered plugin. If it offers to edit, try typing Command-I to see the about page (I for information).
Once you find a site you can guess that the site has documentation titled About Foo Plugin. Type a link to that and click it.
The documentation for a plugin will say where to find its source. This is likely a GitHub project with contributors and administrators ready to talk with you. If you find them trustworthy you can probably trust their code.
Look for Their Inventories
You may find inventories of plugins made to work together and ready to be installed from a Plugmatic plugin inventory. See Managing Lists of Plugins
Look for Their Commuities
You can also start by looking for communities making wiki plugins and see what they have to offer. Anyone distributing a wiki server is likely bundling plugins for it. We'll list communities that we come to know and trust on this site.
• Plugin repos at FedWiki. search 
• Plugin repos by WardCunningham. search
If you're writing plugins, please let us know so we can try them out and add you to this list.
Make Your Own
If you add plugins of your own design to the servers you operate then content created with them can still flow outside of your realm. The json format used for exchange will carry the data and it will show well when it returns. Outside your realm viewers will know they are missing something and can simply move to your sites as origin to have the experience you intended.