In the end of What Lies Beneath (Strangers and Freaks). It is available at the docks parking lot before you approach Abigail after collecting all of the submarine parts, but it is locked. The only way to obtain it is to kill Abigail before she gets in the car. The Asea will become unlocked afterward.

In the GTA Online tutorial race "Mall Or Nothing" the Asea is one of the selectable vehicles. If you win, you can keep the Asea.
Trivia

The Asea has decent top speed and great acceleration, on par with its brother. Handling is superb. It can easily do both bootlegs and J-tursn, but unlike the Futo, it has more grip and is easier to control. Crash deformation, however, is low due to its weight as it can only withstand 2-3 hits before the wheels are jammed.