In Gran Turismo 5 for the PS3, there are a couple different ways to find the rare Formula GT race car.
First, it will show up in the used car lot randomly, so check it often. The used car lot changes about 20% of its vehicles after every race, and after every license test. One trick to speed this process up is to load a license test, and then load the next test without doing the test. This will make one day go by, and some of the cars in the used car lot should be different. Every five days all of the cars in the used car lot should be different.
The other way to get the Formula GT is the online car lot. It should be available all the time in the online car lot for 4.8 million dollars, so you had better start saving if you want to buy it.