You can access the newlib source in a few ways:
You can use the gitweb interface.
git clone git://sourceware.org/git/newlib-cygwin.git
git clone sourceware.org:/git/newlib-cygwin.git
Source snapshots are available as gzipped tar files via anonymous FTP from the newlib ftp directory or http. New snapshots are packaged and released from time to time. Documentation snapshots are also provided.