In an out-of-air emergency there are really only two things that matter-your next breath, and where it's coming from. An emergency air system (bailout bottle, bailout cylinder or emergency gas supply) is a scuba cylinder carried by a scuba diver for use as an emergency supply of breathing gas in the event of a primary gas supply failure.
Choosing among the available emergency air systems is an exercise in trade-offs. Small, compact bailout cylinders are easy to travel with, easy to use, easy to share and they can be stowed almost anywhere, but depending on how deep you are, may only give you just enough air to kick for the surface or to find that wayward buddy. Bigger emergency air cylinder systems provide more options in that they may allow you to make a slower ascent, but they also take up more space on your gear, are harder to travel with and can create hydrodynamic drag.
The Scuba Doctor dive shop can supply the H2Odyssey Extra Air Source kit solutions, or help you to build a different setup to suit your particular requirements.