http://www.azulsystems.com/products/zulu mentions the code is released on GPL + classpath exception but I didn't see any pointers to where the source code is available ?
Is it built from a branch in the public openjdk source tree or somewhere else ?
You may send a request to azul (underscore) openjdk (at) azulsystems (dot) com for a source snapshot bundle. Please indicate which Zulu version(s) you require. A download link will be emailed to you.