I've been making my own shiplap for many years, and it's not difficult to do.
Put dado blades on your table saw and cut a rabbet that takes away half the thickness of your wood. Flip the wood and make a rabbet on the other long edge. If that sounds confusing, a picture is worth a thousand words:
Your boards then overlap and can be made in any thickness and any width, depending on the use.
I used shiplap on the back of my buffet that I posted about here on my blog in 2010.
Shiplap allows wood to expand and contract with the seasons. The gaps between each board will widen during dry weather and get narrower during times of higher humidity.
Have you made your own shiplap?