On Google Chrome I have an extension known as Font Changer which can previously could be accessed on the Chrome Web Store via it's "View in Chrome Web Store" link present within the extension's settings, however now it only yields a 404 Error page. Are there any ways to acquire any unlisted addons, other than hoping that the developer republishes it?
There is no universal way to acquire unlisted addons. Someway you should get the installation or source files but you should be aware that they might not work on the current version of the operative system Chrome.
Be very careful on selecting sites that archive software installation files as some of them modify the original files or might require to install other software that might be no good.