Hi -
You can sometimes find cases for sale used on EBAY in decent condition. Or dead Amiga 4000 guts still in a nice non-yellowed case.
I would suggest, however, that you upgrade to a nice El-Box tower case for your A4000. That gives a buff power supply and plenty of space for future upgrades plus you can add a mediator PCI daughterboard as well really expanding your possibilities.
While the A4000 case is cute an novel looking (especially if it has a video toaster sticker on it) a tower case is really worth the $$.