Other computers. And I'm not trying to be funny - that's where I get mine. We do a lot of repairs and upgrades for customers etc. and we keep the drive bays and blanking slots that we take out of machines when we put in something. That's the only reason we have lots available...
You could in theory try the case's manufacturer though...
Microsoft MVP : Visual Developer - Visual Basic [2004-2005]
Dont though all that away when you build your system. What i normally do when building a new system is keep the motherboard box and put all spare parts like IDE cables/drive bay covers/un-needed cables and coords/stupid manuals in the mobo box incase I need something.