From September, 14th, it's possible to download the developer preview of Windows 8, at this address: msdn.microsoft.com/en-us/windows/apps/br229516
Don't download the one with the developer tools: it's mostly useless for common people, I suggest to download the 32bit on because it works better in emulators. (I don't recommend to install it in a real computer: data loss ahead!)
After the installation, I bet that you want to disable Metro, the new graphical interface: here is how to do.
If you want to run Windows 8 in VirtualBox you need to follow some directives, otherwise the installation won't be completed.