Get Books plugins are very simple just look at a few existing ones to get the idea (look in the src/calibre/gui2/store/stores subdirectory in the calibre source code). While you could make a standalone plugin, it is probably easier to just run calibre from source as described here:
https://manual.calibre-ebook.com/develop.htmland add your plugin to it. Once you are happy with it, you can submit it for inclusion into calibre.