zlib 1.2.6 is still available on sourceforge: http://sourceforge.net/projects/libpng/files/zlib/1.2.6/zlib-1.2.6.tar.gz/download
same with libpng 1.5.9: http://sourceforge.net/projects/libpng/files/libpng15/older-releases/1.5.9/libpng-1.5.9.tar.gz/download