I've bought a few hd dvds from inet and fortunately not had any broken cases from them. I suspect if you need to return any the postage would be hideous.
If it helps i bought some blank cases from a site called svp.co.uk, they cost me about £4 for 5, they are listed at 23p each, most of this was postage.
They look really good virtually identical to the originals, especially colourwise.
You could bite the bullet and just buy some new ones but i'd probably email inet first and see what they say, for delivery i've always found them excellent so you'd hope they would have some fairly decent back up.