Where is the latest source code of man command for linux?

You can usually query your distribution to see where sources come from. For example, I'm on Fedora, and I can see that the man command comes from the man-db package:

$ rpm -qf /usr/bin/man
man-db-2.6.7.1-16.fc21.x86_64

I can then query the man-db package for the upstream url:

$ rpm -qi man-db | grep -i url
URL         : http://www.nongnu.org/man-db/

And there you are, http://www.nongnu.org/man-db/.

You can perform a similar sequence of steps with the packaging systems used on other distributions.

On Debian based distributions, same like Ubuntu, you can find and download source code like below:

$ which man
/usr/bin/man
$ dpkg --search /usr/bin/man
man-db: /usr/bin/man
$ apt-get source man-db

This will put the source code in your current working directory.