(This question is referring to my suggestions for painting en plein air or outdoors on location. You can read more about the kapablock by visiting my website tips for Outdoor painting gear page 2 .... And don't miss page 1 for more tips.)
Kapablock can be found at plastic suppliers such as Tap Plastics or Calsac Plastics in my area.
It can sometimes be found where ever they sell advertising supplies. It is also used for building the partitions between display booths. Some lumber supply store may carry it.
If you can't find kapablock look for the 1/2 in gator board. It might be easier to find.
Good luck! And enjoy painting outside!