The Ford GT40 can be purchased from the Torn Docks. It can also be obtained through the "Hijack a Car" and "Steal Car from Showroom" Grand Theft Auto crimes. It is obtained about 5% of the time (or roughly one time for every 20 successes) from "Hijack a Car," and about 31% of the time (or roughly one time for every ~three successes) from "Steal Car from Showroom." 
The Ford GT40 cannot be found through city finds but can be traded between players.
- Damaster, GTA Crime Stats (14/02/23)