Looks like PETT has the old OD backs for the military sector. Of course they have new civilians ones as well, but I like the original OD version. Below are some links on where to find them and the pages on PETT's site.
http://www.armysurpluswarehouse.com/wholesale-or-bulk-case-items/military-issue-wag-bag-case.html
http://www.cleanwaste.com/markets/military
http://www.cleanwaste.com/markets/military#pop
http://www.armysurpluswarehouse.com/wholesale-or-bulk-case-items/military-issue-wag-bag-case.html
http://www.cleanwaste.com/markets/military
http://www.cleanwaste.com/markets/military#pop