Successfully complete bike school with all gold awards to unlock the NRG-500. Alternately, go to the docks at the Easter Basin area in San Fierro. Go to the yellow crane near the ship (you will have a mission here). On the other side of the yellow crane is a ship dock that is not filled with any water, and has a hanger-type shelter covering it. Inside the shelter near the far wall is an NRG 500. When you get on it, a mission will start. Just ignore the icons, and after a few seconds the mission will fail but you can keep driving the NRG-500. You can return here after some time to get more NRG-500s because they continue respawning. In Los Santos, go into the yellow territory on the right side of the map. There is a L shape building, and inside is a parking garage. There is an NRG-500 on the second and top floors. The one on the second floor has two tail pipes, and the one on the top has one. Also, there is a Uzi at the top floor.