Can anyone tell me where I can find & buy the type of green pickle served with Kao Soi.. 

In any and every supermarket and local fresh market.  It's pickled mustard greens, and in its 'whole' state looks like this.  In the supermarket you'll find it in small plastic bags in the refrigerated shelves near the Kimchee and other Asian preserved veggies. In the fresh markets, it's usually like the picture in larger tubs of brine.  Shouldn't have any difficulty finding it.

 

In Thai its called "pak gard dong".