I've never seen them....But......What you could do is cut the tube and then after drilling the blank just glue the two pieces of tube into the opposite ends.
Really no need for tube in the middle of the blank.
If you talking about a closed end pen then you don't need a longer tube at all.
You can get long 10mm tubes that you'll need to cut yourself. I bought a pack of cigar letter opener tubes to use for my closed end barons. They are 10mm and quite a bit longer.
I just last week verified this with a dig caliper, and the lower baron tube is identical to the cigar tube. the top one is the oddball, and if anybody knows a source for them, I would like to know.
Thanks all for your help. I went to A.S. and they seem only to have the cigar tubes in the long lengths. I think next time I order I will just purchase an extra set of the Baron tubes and make each half of the pen about a 1/4" longer. I want to make this kit more user friendly for people with large hands. Whats your thoughts?