I made a post in the "Odd Treasure Troves" thread about zoos/wildlife parks being a possible source of firearms. Many zoos around the world have firearms trained personnel to take care of any dangerous animals that escape and so on.
These places can provide a small quantity of civilian shotguns and rifles but most people don't associate zoos/wildlife parks with a cache of firearms.
This is the article from The Firearms Blog that I linked to in the other thread, it has much more detail than I've mentioned here.
http://www.thefirearmblog.com/blog/2...irearm+Blog%29