When you buy a pen kit, the tubes for that kit are included. If you happen to mess one up........er, I mean, if you should misplace one of the tubes, I believe all of the suppliers sell spare tubes for the individual kits. Additionally, I know that PSI and Woodturningz sell 10" tubes in 7mm, 10mm, and (I think) in 8mm. Having said this, however, I would strongly suggest that, at least for a while, you purchase only precut tubes, so that you can be assured that the ends are "square" with the walls of the tube. If you DO choose to buy the longer ones and cut them yourself, be cautious that you don't crush the end of the tubes, that they are square to the walls of the tube, and that you deburr them so that the components will be accepted.