How to get source with apt-get source in Ubuntu?
The source lines look exactly the same as the normal package lines, they just say deb-src instead of deb.
so just copy the ones you have now and change them to be deb-src.
Update - You may also need to apt-get update after adding the deb-src lines.
If you open /etc/apt/sources.list file, you will see commented out lines (starts with #) which have "source" on them.
Uncomment the one for universe (thats where pure-ftpd is). Then save and close the file.
Run:
> sudo apt-get update
And then you'll be able to download the sources.