As far horse manure, I was wondering if the Seattle Police Mounted Patrol at the south end of Westcrest Park in the Highland Park neighborhood might be a possible source, and in doing a quick search, came up with this, from nearly a year and a half ago, that indicates it would be:
http://www.seattlefarmschool.com/2014/12/garden-now-mulch/
You may want to call, stop by there, or see if you can ask via their Facebook page if you are on FB: https://www.facebook.com/FriendsoftheSeattlePoliceMountedPatrol/info/?tab=overview
Mike