High-gain omnidirectional antennas for Wi-Fi mesh
First you need to understand what is meant by "high-gain" and "omnidirectional".

A true omnidirectional antenna is one which radiates equally well in all directions. Think of the sun, it sends out light equally well in all directions, very little of which actually reaches the earth.

Such a true omnidirectional that radiates equally in all directions is known as an isotropic radiator. Wikipedia has a nice writeup at http://en.wikipedia.org/wiki/Isotropic_radiator

Now here on earth most of the time we are more concerned in sending the signal out horizontally rather that straight up into space or straight down into the earth. Any "gain" antenna achieves it's gain from reducing the power sent in some directions and re-directing it into other directions. It is important to remember that an antenna does not create power, only focus it in certain directions. Think of it like a flashlight which uses a parabolic reflector (just like a satellite dish) to favor some directions over another.

So what you are looking for is not an isotropic antenna but one which radiates well in all horizontal directions. This is achieved by reducing the energy sent out in the vertical dimension. The pattern would look like a pancake parallel to the earth. The flatter the pancake (or radiation pattern), the more gain in the horizontal dimension. 

All professional grade antennas made by reputable manufacturers will have technical data sheets showing plots of the radiation patterns.

Of course you may be able to easily construct an antenna on your own if you are handy. There are lots of plans on the internet.