Chinatown Detective Agency

This will come through natural progression of the story but if you want to grind it out early you can do.

Personally I got this once I had done the first 3 missions as you should already have visited around 5-6 cities.

I then saved the game and proceeded to the airport to catch a flight to a new city. Once there I straight away got a ticket for a new flight to another city and so on until it unlocked. Tickets seem to be a standard $550

I then reloaded my save and got the money back for the flights and carried on with my game.

You should have around $28k at this point so could even do the 30 Flights achievement and then reload the checkpoint 👍