Get plugin download URL from slug

I forget exactly if the plugin slug is a reliable method to get the url, it might be in most cases but not all due to how plugins are named.

You can query the api @wordpress.org for the plugins xml file which also contains the download link.

For example:

$plugin_slug = 'akismet'; 
$return_plugin_info = "http://api.wordpress.org/plugins/info/1.0/$plugin_slug.xml"

It will return an XML file you can parse for the download link, which is in <download_link type="string">the link...akismet.zip</download>