I buy the boxes for something like $1.25 or such at liquor stores. I used to spend tons of time running around and stopping at every package store i saw searching for these specific patel boxes, but then I came to my senses (such as they are) and gave the lady that runs the humidor at my local store a pen, and now she saves them for me and calls whenever she empties a couple.
I like these because they are shallow, and the inside of the lid around the sticker is flocked in black. They match the black pen tray liners I got from a vintage pen dealer online, but there are several links to his page around here, just search for tray liners or something like that.
Honestly I like these liners for slimlines and euro's better than for jr gent's or larger. I wish I could find some with wider grooves. I have been telling myself I need to just bite the bullet and set up a angle sled jig on my table saw and make myself a few board feet of liners grooved to fit the bigger pens. Of course, that takes time, and that is almost as rare as cash around my house. Then again, I keep telling myself that the bigger pens really need to be displayed and sold in individual boxes. Which I need to make cause I can't afford to buy them, but that takes time again. It's a vicious circle