Here's a blog article on the "DC Implosion" that shows many of the DC house ads from 1976-1978, and a few from before and after that period.

http://www.dialbforblog.com/archives/252/

For any who haven't seen them, there's also a number of pages from the two-volume CANCELLED COMIC CAVALCADE, where the material from the 17 cancelled/unpublished DC issues (30% of DC's total comics line at that time!) were printed in Xerox form.
(Although many of the stories later were reprinted in the 1980's in regular comic a few years later. The Black Lightning stories by Nasser for example were later printed in WORLDS FINEST, the OMAC backups by Starlin were printed in WARLORD 37-39.)