How to find old WordPress plugin? Look for it in the repository! Seriously!
I was looking for XML Google Maps, as one of the best plugins out there for google maps, routes and stuff… so after I couldn’t find it through the search on http://wordpress.org/plugins, I checked if the repository still remembers it: http://plugins.svn.wordpress.org/xml-google-maps
Obviously it does. You can even find the last free version of WPML 😉
Another pretty useful resource that you might miss about a plugin is the support forum. You still have it, though! Here it is: http://wordpress.org/support/plugin/xml-google-maps