Vivvix Formerly known AdSpender
Tracks millions of brands across 18 media and ranks top spenders by industry, parent, brand, etc.
WARC : ideas and evidence for marketing people
Provides access to advertising and market research conent from a variety of sources. Also includes case studies on successful marketing strategies
Good sources for the advertising and marketing literature. Try this search in either database to get started: (marketing or outreach or advertising) and (museums or "cultural institutions")