A lot of people, myself included, keep their original packaging in storage for just this sort of contingency. I would ask around locally, first. Maybe you'll find someone who can "loan" you their packaging. Second I would call any local retailer who sells refurbished iMacs and see what they use when they ship. Maybe they'd sell you the packaging, or tell you where to go (to find it, that is). Third I'd contact online retailers of refurbished iMacs w/the same question.
Good luck.