Thanks to the help of a new friend, I was able to find some more letters. The letter in The Harvest Counties was near the Goldspur Farm, in the paddock with the horses. I found a letter in Valedale that was right in front of the clothing merchant (although locations tend to differ from person to person). There's a letter near the Jarl's Tomb, right before Fort Pinta. The final one that I've found was in Fort Pinta, near the salon next to the disco. There is supposedly a total of nine, so this should be all of them! Thanks for reading and happy trails! This is what it shows when you find the last one and turn it in. There's no need to go back to Derek after you've completed this. He does not give a followup quest.
Veronika Starstorm's musings and helpful pointers on the world of Star Stable Online