I asked the same thing a while back. The best answer that I got was to go to a hobby store like Hobby Lobby or some such place. In the bead section they have the perfect plactic cases deep enugh for buttons. No foam though. I am using cotton balls for now until I can make some foam inserts. I got a pack of like 20 for under 5.00 in the craft section of our Wal Mart. I hope that this helps.
You might contact the good folks at Sgt. Rikers and see if they can supply what you need. Here's a link to the button cases and holder I think you are talking about.