Lots of good info already posted. I'll add thing to be aware of at truck stops. In some states, Oregon being one of them, diesel is sold either with road tax or for truckers w/o taxes but they pay weight/mile tax. So..... be careful at truck islands. you don't want to stop at the island that needs a PUC permit to fill, not good, cashier is not going to like it, or you'll find it won't dispense because you don't have a permit number to enter into the system.
You'll know it is one of those islands because the price/gallon will be about $.25 or more less than regular diesel.
At least on the west coast I'd say 80% or more of the stations have at least one diesel pump. Finding it is not an issue, and in small towns with farmers, you may find more places to get diesel than gas. some co ops have lots of diesel islands and not many gas islands.