Most newspapers, magazines, & broadcast media provide demographic data about their audience for advertisers in an online media kit. Go to the company/publication website & find the link on that page for Advertisers (often at the bottom of the page). One can usually find a media kit with detailed information about market and readership. See the Atlanta-Journal Constitution Media Kit OR NYTmediakit as examples. In Google, try searching for the publication name and media kits, or advertising rates. E.g., "business week" ("media kit" OR "advertising rates")

To find the Top Lists of magazines or TV channels -- do a simple search in Google for something like "top magazines for teens"

WARC (World Advertising Research Center) has all kinds of helpful information like using TV ads well (effectiveness, length, context and positioning, scheduling, audience measurement).