http://www.spareair.com/product/models.htm

As far as purchasing one, the last I checked they ran about $300.00.
Our store http://www.visibilityunlimited.com used to sell them but because of the high cost, not too many people were interested enough in them,
You can buy them but in order to fill them with breathing air, you would need to be certified in scuba diving.
If what you are talking about is not related to a spare air, then it is probably a Scuba Regulator.

http://www.visibilityunlimited.com/page/scuba/CTGY/Scuba-Regulators-Octopus

The regulator can, again be bought without a Scuba certification but since the scuba cylinder that it attaches to needs to be filled with air, you would need to show a scuba certification card before it could be filled.

The spare air only lasts about 30 breaths at the surface. The amount of breaths is shortened the deeper you go underwater,
At 33 foot of depth the number of breaths go down to 15.