Do not ship your iMac using only the thin outer box. That's OK for Apple Computer, who can afford the risk. You cannot take that loss in the event your iMac becomes damaged.
I have shipped a couple iMacs through a UPS Store. If they don't have a larger box, then they will make one. But you must ship the item through them to get the box.
When you ship an item that was packed by the store you can recover the complete amount for which it is ensured. This isn't necessarily the case when you pack the item yourself. Nor will you be able to collect if you use only the thin box and ship the iMac via UPS.
Another idea is to visit other stores. If you live near The Container Store, you can find a good selection of boxes on their shelves. Or you could order one online.
http://www.containerstore.com/browse/index.jhtml?CATID=62516
Mac OS X (10.4.8)
Mac OS X (10.4.8)
Mac OS X (10.4.8)
Mac OS X (10.4.8)