Kevin
You can get 25mm pen blanks at http://www.ukpenkits.com/Pen-Blanks/Pen-Blanks-25mm-diameter but there is not as much choice but they are nice you can also get them in 20mm.
But as others have said for a Jr Gent 20mm is ok, you will only be spending time turning away all that extra plastic. As long as your drilled hole is central (at both ends of the blank) you will be fine.
Remember that you are likely to have in places less than 2mm of wall on your finished acrylic pen & on some acrylics they become translucent & you may see the brass tube, so you may need to paint the tube first.



