If you work in an office or know anyone that does, as above ask IT dept if you/they have one, they usually have server/workstation boxes which are pretty sturdy.
Sometimes you can muster up something from a supermarket or in local village shops, I was seriously paranoid about sending a Corsair 650D case in the post, lost the original box, couldnt find anything ANYWHERE within time, even from work....
Ended up getting a giant box from the supermarket that they had in from Quavers, box was huge, added some other cardboard inside to re-enforce it, used poly and bubble wrap, jobs a gooden
Some people will let you rummage through their cardboard skips