I think it unlocked after I completed the first non prologue mission. I got a notification about strangers and freaks missions being available then I found it near the LSC in the desert (the one franklin can buy) in the motel down the roads car park. Though as stated above it may require a certain percentage or something I can't remember sadly.
The Imponte Duke O'Death is an exclusive vehicle for returning players of Grand Theft Auto 5 on PS4 and Xbox One. It will appear as a Random Event after you complete the first Tonya Strangers & Freaks mission Pulling Favours with Franklin - found opposite his aunt's house at the beginning of the game.