Contact CLC, they do sell the material for inner rails.  When I ordered my inner rail kit, the material (1/2" wide and 7/8" tall) came nicely scarfed to make two long pieces first, then install. There was plenty to make all the inner blocks.  Dont make too many mistakes and you should have plenty of material.  If you plan ahead you can even  use one of the scarfs to kill into the transom nicely.

There is a discussion somewhere on here about using the same size stock as on the outer rails, but that can look too massive...1/2" wide is a nice look.

Many pictures of mine on each side of of this one:

https://picasaweb.google.com/114717787929554738224/BuildingWoodenBoatCLCNortheasterDory?authkey=Gv1sRgCKCBzczm_aKs2wE&feat=directlink#5992553614103239634

Curt

   If you check under options for the boat the inwale kit is listed. $349.00 seems to be the magic number.

   Interesting..I got my materials (inner rail stock only) before it was listed as an optional item to the boat.  I see the kit comes with a new (2-piece) breasthook, new knees and oar lock socket risers.   I adapted the breasthook and knees that came with the boat myself and did a test on oar lock heights and decided I didnt want risers and it all worked out great.  FYI

Curt