If an article includes an ISBN number, the Mediawiki software creates a link to page Book sources, giving a small list of sites that sell new and used books.
Without editing the PHP, it should be possible to do this in the wiki itself on some page. How to alter this list to include other book sellers and book finding sites?
You can do that by editing (or creating, if it doesn't exist) the page Project:Book sources (or a translated version of that name, if the language of your wiki is not English).
In the source of that page, MAGICNUMBER will be replaced by the ISBN. You can look at the version of this page on English Wikipedia for inspiration.
