Making bushings of different material is not as difficult as it might seem at first. Once done, you have a new tool, actually 2 new tools - a set of bushings and the ability to make your own going forward. Making your own is a great experience in precision making. Be sure younhave a good set of calipers.
I've made them out of Delrin with pretty good success - you just need to verify your dimensions. Grainger or McMaster-Carr sells it in different diameters and length. Turns real easy.
Or, learn to turn between centers without bushings and measure the ends as you go to match the hardware you will be inserting into the tubes. If you are making only one or two of the said pen kits this might be a better use of your time.
Or convert the metric sizes to imperial and look them up on the bushings and tube app and see if you can order them that way through Exotic Blanks or some other distributor.
I've taken to matching existing bushings with the diameter of the kit tube. I ignore the OD of the bushing and rely on ye olde calipers. I do make sure the diameter of the bushing is smaller than the needed turned OD.