To use any of the free liveries mentioned above, you need the official A2A model. It is available through several retailers: Accu-sim C172 Trainer (P3Dv2-5) - A2A Simulations
FlightGear.org (The entire sim is free)
However, obtaining this software for free usually involves significant risks to your computer and your sim's stability. Below is a guide on why this aircraft is so highly valued and the best ways to acquire it safely. Why the A2A Cessna 172 is the Industry Leader a2a cessna 172 download free