I don't think you will have much luck finding the same tubing - pen tubes are oddball sizes. You will need to buy them from someone who sells pen making stuff. Some sizes come in 10" lengths.
In the library there is a tube and bushing reference. It is worth the time to find (I downloaded it and keep it on my desktop - always handy). It has the dimensions for most of the kits, and I would bet the Gentleman's is in there.
For tubes that don't come in 10" lengths, what I do is look for another kit that uses the same diameter tube, but is longer, then buy replacement tubes for that other kit. For example, Woodcraft's cigar letter opener tubes are the same diameter as the cigar pen, but are a little longer. I buy these instead of replacement cigar tubes, then I can trim to the length I want.
