I am sure you are aware aero bars are used primarily on Time Trial and Triathlon bikes.
These types of bikes are not usually your every day riding bikes. They come out when there is a race to be won.
As for the rest of us with out this luxury here is what we can do.
There are several makers of aero bars. They make a variety of different styles. The kind which works best with bikes having existing handlebars is called clip on aero bars.
These types of bars actually fasten to your existing handlebars. They come complete with fastening hardware, arm rest pads and the bars themselves.
There is however different styles of bars or extensions you can get.
There are three major styles to choose from.
The first style is a straight bar. No bends of any kind.
The second style has a bend in it and is called the chicane. This one looks like you were drawing a straight line and someone bumped your arm. So there is a slight jog in it.
The third style looks like the end of professional ski pole. This is the reason it is called a ski tip extension. The ski tip extensions have a more drastic bend than the chicane extension does.
Each extension has its own set of benefits.
The ski tip and chicane will allow you to get your hands closer together. Being able to do this provides you with a more aerodynamic position. This ability helps you to streamline your body position on your bike.
The straight extension provides you the ability to change positions on your bike. It also allows you to get more aerodynamic than being in your drops. If your primary aim is not aerodynamics then these extensions are perfect.
All the extensions have there good points. It is up to you to assess your situation and desires with what each extension offers. Once you have done this you will have a better idea of what you would like to get.