Shops like 3Rivers carry the boxes and dividers but with shipping and minimum quantity they aren't inexpensive. If you have s shop in your area where you may be able to get just what you need that would cut cost.
Failing that I suggest priority mail tubes, the longer of the two sizes available. They're free at the local post office and I can fit a dozen arrows in with no problem, 6 up and 6 down, no divider. Use some newspaper so they don't rattle. If I'm trying to impress someone I use regular arrow boxes and dividers, but otherwise these priority tubes work great for both cost and effectiveness.
Guy