"Find a Dinka Double-T" is an objective in the Strangers and Freaks missions from The Epsilon Program in Grand Theft Auto V.
There are 2 ways to get this car:
If you have $,000 to spare, you can buy it from the San Andreas Super Autos. It will be delivered directly to Michael's garage and then you can take it to the Epsilon marker found in Vinewood Hills
If you are more strapped for cash:
The Dinka Double-T can be found by the Lifeinvader Building in the parking spaces by it on Boulevard Del Perro.