I guess the obvious question, would be do you have the original box. Which I assume your answer is negative.
I would search around my local area for a school or small business that may have iMacs and an old box that you can have.
Apple provides shipping boxes for their computers to get fixed. Maybe you can purchase one of those.
If the options above don't work, get a box that allows for lot's of padding and is durable so the box doesn't break during shipping. Use supporting tape around the box.