agency has 9 distinct synonyms spread across 3 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
an administrative unit of government
“the Central Intelligence Agency”
a business or organization that provides a particular service, especially the mediation of transactions between two parties
the state of being in action or exerting power
“the agency of providence”
the state of serving as an official and authorized delegate or agent
thing or person that acts to produce a particular effect or achieve an end
“a means of control”
Synonyms, antonyms and the broader and narrower relations for agency come from Princeton University’s WordNet. Full sources and licences.