A while back I found a bunch of boxes used to ship computer cards. They even had foam egg crate padding. As long as you bag the gun in a ziplock, they're fine. I don't trust foam against gun metal, but I do if its bagged.
When I ran out of the card boxes, I lucked into some nearly identical boxes sold at Office Despot or Max, I forget which. They weren't cheap, but a lot less then $6 each. I'm guessing a buck or two. But by then I'd been finding and saving all the eggcrate foam I could.
I'm going to suggest you tailor your box to the job you expect from it. My needs were just long term storage, so a box with a plain end I could use a magic marker on was great. No one but me was supposed to see them. But like most plans, mine failed and I ended up taking some of them to gun shows. I just improvised and never put the box on the table. All the vultures saw was a bagged gun coming up above the table.
The real advantage is the supply store advertised them as shipping boxes. To that end, you could buy them in different sizes, some long enough to put in 6 1/2" guns. Back then my safe looked really clean and well organized. All the shelves filled with similar boxes, labeled so I could find things. Life was good. But the plan didn't last.