Where is the PPA sources config file?

PPAs tend to be added into their own files in /etc/apt/sources.list.d/. Here is an extract from a ls in that directory on this system:

am-monkeyd-nautilus-elementary-ppa-maverick.list
am-monkeyd-nautilus-elementary-ppa-maverick.list.save
awn-testing-ppa-maverick.list
awn-testing-ppa-maverick.list.save
banshee-team-banshee-unstable-maverick.list
banshee-team-banshee-unstable-maverick.list.save
banshee-team-ppa-maverick.list

If you're trying to move these to another system or back them up, just using these files is not the best idea because these files have version specific strings in them. Look at my answer on another question. It will generate a list of PPA strings that you can add back manually (or with a little script that just loops the list).