Gas Buddy is not only great for finding fuel stations, it's also a really good reliable map program. I use it over Google Maps when traveling. I know Google does the same thing, but I really like the instantaneous "here you are on the map" dot! And then it shows you the surrounding fuel stations.
Diesel is diesel. It doesn't come in different grades like gasoline. The only variation is #1 and #2 diesel. #2 is used for home heating and not for vehicles. You won't get it at a fuel pump anyway. So any fuel pump you use (truck or car island) will be good.