I have been collecting PSA campaign ads on Pinterest, organized by category.
Other great places to find ad campaigns are:
Osocio – Nonprofit advertising and marketing for social causes
The Communications X-Change – online library of materials contributed by organizations and individuals around the world who are working to end violence against women and children
Ads of the World – huge ad database searchable by media, country, region, and industry
Coloribus – even bigger than Ads of the World, with greater search capabilities and over a thousand new ads added daily