I imagine if you went to the effort of presenting yourself as a legitimate business by providing some advertising and financials, you could request the media package from any media outlet.
I have done this in the past, legitimately, with magazines and the demographics and breakdowns are usually right in there, complete with sub-headings.
I know someone who went the infomercial route and she seemed to have a lot of information about target audiences by geographic locale and time of day. The infomercial companies run test ads first and these are planned, I gather, to test the product against known demographic points.
I found it interesting that places like DFW and San Antonio seemed to be favored TV ad test markets. Back in the day, it was places like Milwaukee, WI and Peoria, IL. The product under consideration was a fashion accessory.
Nielsen makes you pay for it. In fact, that's really how they make their money, off prospective advertisers researching where the best "fit" is for their advertising dollars.
Nielsen will gladly release their numerical viewer data, but they won't release the more interesting demographic data, because that's their proprietary hook.
It must be powerful data too. As an example, while NCIS doubles or triples the number of viewers for Heroes, Heroes still commands a premium advertising rate, much larger than what NCIS can command. IOW, Heroes has far fewer viewers, but generates MUCH more money for NBC than NCIS does for CBS.
I got interested in this when CBS canceled "The Unit". I thought, why are they canceling a program that outperforms virtually every show on NBC. The answer was in the show's demographics and the ad rates it commanded.