There are a few informants that appear in City Hunter and will be listed here.
The boss of Nightclub Melody who also acts as an informant for the police and Ryo. Ryo helps out with issues here as well per request. An elderly man with white hair but one who dresses smartly.
A young lady who is Ryo's informant during the 'Missing Beauties' episode. She seems to have taken a liking to Kaori when she was disguised as a guy.
Another informant who knows Lili. He tipped her about the boutique in the 'Missing Beauties' episode.