Where can I buy an empty box for shipping my iMac 27 overseas?

You could contact your local Apple store and see if they have any you could use.

However, personally, I would go to a shipping service and let them pack it and ship. That way you can insure it against any damages. I'd also demonstrate to the packers that it works before turning it over to them (and video the demo for evidence of it working).

Have you simply checked with the shipping company, you will be using, to see if they sell boxes specifically for this purpose?

Plenty on eBay.

Ebay, Craigslist, Marketplace

Finding any empty box should not be that difficult. How to properly pack your Mac in it, will be critical ... of course, that's assuming that you want it undamaged when it arrives.

Most shipping companies do accept packages that are user packed, but many will not guarantee the contents from any damage. That's why they charge more.

I agree with Old Toad about checking with your local Apple Store, and, even more so, with the suggestion to let the shipping company do it.

+1

Your welcome.

Where it applies, ensure that you are following the current sanctions imposed by Canadian guidelines when shipping overseas from a Canadian province.