Along with doing my own research, I source content from a ton of really great sites.
http://goodvertising.site/ – I’m a monthly contributor here. Great resource for inspiring stories and groundbreaking communication approaches for sustainable and social initiatives.
http://www.sustainablebrands.com/ – Current events and stories about responsible business
http://www.adweek.com/category/adfreak/ – marketing/brand news
http://adage.com/blogs/ – marketing/brand news
https://www.freerange.com/thrive/ – kinda consultancy, kinda agency, cool thought leader in purpose-driven marketing.
https://www.wearefuterra.com/ideas/ – Another cool agency with some awesome ideas
https://www.triplepundit.com/ – sustainability news, focus on business
https://www.huffingtonpost.com/topic/cause-marketing – cause marketing section of Huffington Post. Great for up-to-date content about the topic.
http://www.conecomm.com/insights/ – had the pleasure of meeting Carol Cone herself, founder of this purpose-driven agency (and kind of the “god mother” of cause marketing). Great resource for ideas/insights on the topic.