You can try booting into Internet Recovery Mode using Command + Option + Shift + R which should boot to the online installer for the original OS which shipped with the Mac from the factory. However, we have seen a lot of users report having issues installing the older versions of macOS on older hardware through Internet Recovery Mode. Apple doesn't seem to be keeping their online older installers in working order these days.
While it is possible to download the macOS 10.8 Mountain Lion installer, there are no instructions for creating a bootable macOS 10.8 USB installer.