I looked at dozens of ideas when deciding how to store my markers. I needed them to be in number organization so I could find combinations easily (like you), inexpensive, and easy to come buy (there are some pretty neat ideas that people have made, but that is not for me). The idea that worked best for me and my criteria was the idea that Trudee posted (Thanks for the idea Trudee!). See her picture at
trudee.blogs.splitcoaststampers.com
I did a couple of things different from her. I purchased the size 36 case, not the 72. I also tried grouping crosswise, but some of my markers were in groups of 3, others 5, so I ended up running them lengthwise in the case. Some groups start on one row and end on the next, but I know they are in order so they are still easy to find. I also organize in the same order as Copic does, except that I go backwards to keep them in the same rainbow order as all my paper, ribbons, etc. So - R, RV, V,
VB, B, BG, G, YG, Y, YR, E, then grays. Hope this helps.
P.S. This is the case that Copic makes. The collections come in these cases, but they can also be purchased separately.