Full guide is here.
I briefly write down how I used apt-spy for finding new Debian mirror site. Apparently the primary mirror site I have been became useless. The network connection was so slow that aptitude even failed to update package list.
$ sudo apt-spy update
It downloads README.mirrors.txt that contains Debian mirrors in globe. I found the country name on README.mirrors.txt to save time.
$ sudo apt-spy -d unstable -s KR
It will search mirror sites in (South) Korea and create /etc/apt/sources.list.d/apt-spy.list file. This file contains the fastest mirror site. Aptitude or apt-get uses this mirror site for package update.