Formal:
* Informant: This is a neutral term often used in official or investigative settings.
* Source: A general term for someone who provides information, especially in journalism or research.
* Provider: A broader term encompassing any type of information provider.
* Disseminator: Someone who spreads information widely.
* Communicator: A person who effectively conveys information.
Informal:
* Insider: Someone who has access to privileged information.
* Tipster: Someone who provides confidential information, often in a tip.
* Whispering voice: A metaphorical term for someone who provides information in a discreet way.
* The one in the know: A colloquial phrase for someone who has access to information.
* Knowledgeable person: A general term for someone who has a lot of information.
The best choice depends on the specific situation and the tone you want to convey.