Official download address
https://www.ubuntu.com/download
Zhongke Dayuan
http://mirrors.ustc.edu.cn/ubuntu-releases/16.04/
Alibaba Cloud Open Source Mirror Station
http://mirrors.aliyun.com/ubuntu-releases/16.04/
Lanzhou University Open Source Mirror Station
http://mirror.lzu.edu.cn/ubuntu-releases/16.04/
Beijing Institute of Technology open source
http://mirror.bit.edu.cn/ubuntu-releases/16.04/
Zhejiang University
http://mirrors.zju.edu.cn/ubuntu-releases/16.04/
Other mirror sites
http://mirror.pnl.gov/releases/xenial/