I'm not sure if you need bigger bins but this is what I have for my ammo display. I'm just displaying single cartridges.
Click image for larger version.  Name: Ammo Display 2015 12 06 b 20%res.jpg  Views: 66  Size: 107.4 KB  ID: 64962

I used 1 inch thick rigid foam insulation sheet to make the slots. I cut the slots using a router with a ball end cutter. I used three different widths. I bought the frame / shadow box at hobby shop.

If you haven't already checked them office desk drawer organizers may work if you need bigger compartments.

Good luck on your search.