Characters start with the medallions only, and will have to buy/trade for the journals from a particular vendor located in each town (usually near the marketplace, or in Coba's case by the tavern). The vendor tends to stand out and is only selling these books for basically nothing (a few gold, a pretty common thing you can find outside, etc.), and will explain what they do if characters ask, and also how to use them (that you have to sign the inside, that writing will show up for everyone who has one, and that you can use the medallion alongside it to make your words show up that way, etc.)

Hope this makes sense!