This car can be obtain during the Micheal’s side mission for Epsilon organization. To start this side missions, pull up in-game mobile as Micheal, at internet page type Epsilonprogram.com, then press the “Evaluate your identity” option and complete it.
Micheal will receive a new contact in Raton Canyon shortly after you turn off your in-game mobile. Completing the mission objectives leads you to the point where you should drive 5 cars to Epsilon garage. One of these cars is Declasse Tornado. This is not a rare GTA 5 car, and some of the known locations are:
Morningwood Garage, south from nearby tennis courts, Morningwood
Domestic Japanese European garage, south from nearby Police Station, Central Vinewood