“Agent”, OED says, could mean “an active and efficient cause, capable of producing a certain effect”; “agency” could mean “a thing or person that acts to produce a particular result”. It seems they are synonyms.
Then I come across two related phrases: moral agent, moral agency. “Agent” in “moral agent”, though means simply a subject, an “actor”, can be understood in a sense as a cause of an act.
Since “moral agency” is used to mean the inner dynamics of the moral agent, “agency” is to be understood as “cause” too: what makes the agent act morally? To treat them both as meaning “cause” cannot justify them representing two things different in order.
Therefore I doubt that maybe “agent” refers to the causes which are of natural kind, such as human or corporation, while “agency” can mean the inner cause that explains how the causes work as a cause. Sorry, this may be a very immature thought, please feel free to correct me!