Legal Question in Criminal Law in California

I can't tell from your question whether you're looking for a licensed private investigator, or an expert witness. If you are asking about a real criminal case, the first step of course would be to retain a competent attorney and to then consult with the attorney about which investigator or expert witness to retain. Statements you yourself make to the detective could be admissible in evidence against the defendant in court, so work through the attorney.

If you're a screenwriter type, I suppose you could search for [gang expert detective] and start working the phones.