User:Katsutoshi~enwiki/monobook.js

function externISBN { for (var i = 0; i < document.links.length; i++) {              if( document.links[i].href.match(/isbn=(.*)/) ) { document.links[i].href='http://www.amazon.com/exec/obidos/ASIN/'+RegExp.$1; }   }

}

addOnloadHook(externISBN);