Nightclubs in GTA Online are businesses included in the game as a part of the After Hours update. Any of the ten clubs can be purchased at the Maze Bank Foreclosures website.
You can find a Buzzard on the roof of the NOOSE Headquarters, and it there at any moment of the game. This is a Buzzard Attack Chopper — the version equipped with weapons, not a weaponless Buzzard.
You can get this helicopter without rising your wanted level. To do this, drive through the checkpoint and the gate to the left as fast as possible, and then stop at the ladder leading to the roof of the building. If you stay at the roadway gate or the next gate, NOOSE guards will approach you, and you will get from one to two wanted stars.
Now it remains only to go up to the roof and get a chopper. In rare cases, it may not be there. If you encounter this, come back here later.
In GTA Online
This tip is only relevant for the single-player. In Online, helicopters spawn randomly, and a Buzzard Attack Chopper can spawn on any helipad.