They are all pretty standard but I have seen some places out there that will custom build them for you. I've got, ahem....tractor supply boxes on my....ahem..hayhauler(hehe). I believe they are 36" wide, 18" H, 18" D. They stick up high but then the bed is jacked up 8" off the frame with 4" channel. I've got them crammed full and still need more at times. I've never used top loader style but with what tools I have and what I do I don't think I would like them, to much stuff would be in the bottom or having to drag out stuff piled on top of stuff to get to the stuff underneath!
Seems like there was a place out in Oregon or Washington State, did aluminum boxes for big trucks, built like tanks and from the looks of things very very well made, whoop, found it,
http://www.800toolbox.com/They might custom build you one to your specs, worth a call but I'm hear to tell ya, the 36x18x18 like I wanted was pretty pricey, probably well worth it though.