The Fountain Pen is situated in Osman Grove, that is a hut just east (right) of Emerald Ranch.
To get it to spawn you may want to get Mary Beth’s errand request first. She provides it with for you at the gang’s hideout in Chapter 2 between 8:00 – 20:00 at day. Fulfilling 5 errands is really a requirement of the Errand Boy trophy and achievement. This errand is missable, it’s only accessible in Chapter 2! Restore the Fountain Pen to Mary-Janet at the camp to accomplish this Companion Request.