Rachel Maddow featured the work by ProPublica which you can find crossposted at our own Occupy America here -- Revealed: The Dark Money Group Attacking Sen. Sherrod Brown.
I encourage everyone to just go read their entire report, but I'll share a bit more of the post which Rachel featured in her segment this Monday evening:
The documents identifying Norris as the chairman of the Fund are public because of a Federal Communications Commission rule requiring TV stations to keep detailed records about political advertisers. The files can be valuable, offering a look at exactly who is spending and how much. Until recently, the documents were only available by physically traveling to stations. ProPublica's Free the Files project has spotlighted the issue and this summer the FCC passed a rule requiring the stations in the nation's top markets to upload the files to agovernment website.
The system is far from perfect and has a lot of limitations, but it's a start.
And as Rachel noted, the FCC does now have a searchable site for local stations to see who is running ads in your area. You can visit the site here: https://stations.fcc.gov/.