Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
// [[User:Lunchboxhero/monobook.js]]
// This code will change ISBN links to point to the url of your choice, instead of Special:Booksources. 

var magicURL = "http://www.amazon.com/gp/search?ie=UTF8&field-isbn=MAGICNUMBER&tag=wikipedia08-20";

// [[User:Lunchboxhero/externISBN.js]]
// This code will change ISBN links to point to the url of your choice, instead of Special:Booksources.

function externISBN() {
  var magicRegex = /MAGICNUMBER/ig;
  if(mw.config.get('wgPageName') != "Special:BookSources" && !(mw.config.get('wgTitle') == "Book sources" && mw.config.get('wgCanonicalNamespace') == "Project")){
    for (var i = 0; i < document.links.length; i++) {       
      if( document.links[i].href.match(/Special:BookSources\/(.*)/) ) {
        document.links[i].href=magicURL.replace(magicRegex, RegExp.$1);
      }
    }
  }
 
}
$(externISBN);