Thunder-Rodd
If you didn't get the Thunder-Rodd in the mission called "Grease Sucho", don't fret. If you go to the Policeman's Ball (Grand Theft Auto 3 players will remember this.) in Chinatown, Portland, you can find one sitting there.
Note: There won't always be a Thunder-Rodd sitting there. Sometimes you gotta leave and come back. Also, this is one of the 16 cars you need in Staunton Island for the "Car-azy Car Giveaway".