informant (synonym informer): a person who gives secret information about somebody/something to the police or a newspaper
informant: someone who gives information to another person or organization BUT the example is: a police/secret informant
informant: a person who gives information to the police about secret or criminal activities
So, as I read, informant/informer deals with information passed on to some authorities secretly.
I am not talking of someone who is giving me information secretly or with any bad intention. S/he is just a source of information. S/he is not paid or kept on a job to pass on the information.
I'm looking for a word to describe a person who gave me information about someone/thing but...
S/he's not paid
S/he's not kept for that job exclusively
S/he's does not give me information regularly
S/he's not a professional in passing the information.
S/he doesn't report to anyone.