You can get with:
curl https://github.com/<username>.keys
Replace <username> with the actual username of the GitHub user.
This is useful when you set login permission to other servers. Just save its output into ~/.ssh/authorized_keys. To append it to the end from the command line:
curl https://github.com/<username>.keys | tee -a ~/.ssh/authorized_keys
It can also be done using Github API
curl -i https://api.github.com/users/<username>/keys
For bit bucket you can use the following: (This call requires authentication.)
curl -i https://bitbucket.org/api/1.0/users/<accountname>/ssh-keys