How to have Drush make automatically find libraries download path?
As Molot states, there's no way for drush to know about any arbitrary library; you need to know the source URL to give it. 

For any custom module project, you can use git or file download type to retrieve it from a git repo or an HTTP source.

projects[example][type] = "module"
projects[example][subdir] = "custom"
projects[example][download][type] = "git"
projects[example][download][url] = "git://github.com/jane_doe/example.git"
projects[example][branch] = "dev"

projects[example][type] = "module"
projects[example][download][type] = "file"
projects[example][download][url] = "http://example.com/example.tgz"
projects[example][subdir] = "custom"


The same for libraries

libraries[ckeditor][download][type]= "get"
libraries[ckeditor][download][url] = "http://download.cksource.com/CKEditor/CKEditor/CKEditor%203.4/ckeditor_3.4.zip"
libraries[ckeditor][directory_name] = "ckeditor"
libraries[ckeditor][destination] = "libraries"


Lastly, for local files you can use the file:// protocol in the url property to source the file from the local machine. See the Drush makefile docs for more information.