I'm the member of an equestrian performance group. One of my jobs this year was a Pony Express act. It was one of the most fun sets of shows that I've done (and also very questionably safe, LOL...I performed with a serious case of post-concussive syndrome and wore nothing for head protection but a wool hat).
I have since become completely obsessed and would like to know everything about the life of the boys who were brave enough to live that lifestyle. I would love to see a firsthand journal if one exists, but even well-researched and accurate fiction would be a great find...my performance season ends in two weeks and it might be fun to read or maybe even write something during my break.
http://www.blm.gov/nv/st/en/fo/carson_city_field/blm_programs/recreation/pony_express.html
There are a lot of misconceptions about the Express. They were young men in their late teens and early twenties who road light horses with small "postage stamp" saddles and wore the typical canvas pants and what could be called a "skullcap" hat not buckskin and ten gallon hats. They were in danger of Native attacks and rode through all weather. There isn't much info available in the form of diaries but are some actual reports available of conditions if you dig a bit. The BLM (Bureau of Land Management ) has some archived.
There is actually a Pony Express Museum in St. Joseph, MO. You could try calling them with your questions. When I went to visit, I learned there wasn't really an organized system of preservation for all the equipment and diaries, so they have to rely on the riders' families/descendants to donate stuff. I didn't see any actual diaries but they definitely have the experience researched out.
The other terminus was in Sacramento CA (actually Folsom as there was reliable reliable rail service between the prison and capitol). These organizations may help in your search as there is a lot of local historical interest in the era in the area. Try http://sachistorymuseum.org/ - this is in the same block as the original terminal. The Folsom Historical Society http://www.folsomhistoricalsociety.org/ And probably the best "hidden" historical collection in the state, the Sacramento Libraries' Sacramento Room, which actively collects and compiles docs and ephemera from the region's history. For personal journals and first-person documents, my experience with near-history research - items which people know as "old" or "family heirlooms" - are often found in these smaller, community historical societies as they are generally donations from the families.
Oh, one more: Folsom Prison history. Yup, there is a very cool, small museum recounting the history of the prison which was created and is maintained by the retired guards. They MAY have some information, as the prison was the only reason Folsom existed at the time of the Pony Express. Doubtful, but it's worth a chance.
There is a diary called The City of the Saints and Across the Rocky Mountains to California by Sir Richard Burton. He wasn't a rider, but he traveled the Pony Express Trail by stagecoach in 1860...