Go to Sanrio I found a pen that fits exactly it does not have any sanrio decor it comes in green or blue and is about 3.00 but it work good and it the right fit. It s in the middle where all the pens and pencils are.
I have a Pomme agenda... and I saw a post by another pf'r that had one and she got a Fisher Space Pen!
I ordered one... there are several colors, but the RED is a GREAT match to the Pomme agenda.. it's thin and small.. REALLY inexpensive and for a VERY nominal fee I had my name engraved!! It's TOO CUTE!!!
I use a Zebra pen (I think that's what it's called) in mine and it fits perfectly! I got it at Giant Food but it looks great. You could also do one of those Fisher Space pens, or the Tiffany pens are also really cute.