Hangar 51 is the only place where airlines can purchase new aircraft in Airline Tycoon 2. It is located to the far left on the upper or employee-level of the airport terminal (to the right of the airline offices.) Its shortcut key is: H.
Hangar 51 is also the place to customize aircraft since all aircraft in Airline Tycoon 2 are customizable. To do so, you must first select a plane you would like to purchase, then click on individual parts of the plane to customize them.
The man working at Hangar 51 will also allow you to replicate an existing plane in your fleet. Just let him know that you would like to replicate a plane and select the plane to replicate. The new plane will have the same technical configurations as the replicated plane as well as the same interior and exterior designs that would ordinarily require a visit to the Designer Agency.