There are no bushings for this type of pen available, you have to make them yourself!! This is easily done with corian. Also, you need filler spacers that will fit the tubes to the mandrel you are using (A or B).
I use corian, drill with the appropriate drill bit and glue a brass tube from a pen kit in that will fit my mandrel. Then I turn some of these to size to fit very snug inside the Regency tubes and also make bushings from the same material. I used three different colors so I can differentiate between the tubes for Cap, Body and Section. There was btw an excellent article about this pen in one of the woodturners magazines, written by the Grandmaster of Penturning, Richard Kleinhenz (posting here as "scubaman) and I think there was or is also a link to this on the IAP front page. These pens are fun to make, but it takes a bit longer than a normal kit pen. One of mine, I made into a closed end pen, long before this was fashion (as it is now). I might even have posted a picture here, just do a search for Regency Pen.
Have fun!!!!