You can get OS X 10.8 Mountain Lion completely by the App store, which is similar to that for Apple as OS X Lion was given in the same way. Thank God it can still create a bootable OS X 10.8 Mountain Lion installer from any USB drive, be it a pen drive or any portable hard drive.
You can do a clear OS X 10.8 installs by creating a bootable install drive. Installation can be done on separate partitions and then install OS X Mountain Lion on Macs which lacks the internet connections. We will try doing the process here but u should remember to check OS X 10.8 system requirements for the place of Mac before starting.
If you already posses the Mac OS X Mountain Lion then install the DMG extracted, then don’t do these starting set of steps and start directly to to make the bootable drive as under.
Extract the DMG File of OS X Mountain Lion after Downloading it:
• From the App Store Download OS X 10.8 Mountain Lion
• Click on the Applications - directory and then you should right-click on “Install Mac OS X Mountain Lion.app” and select “Show Package Contents”
• After opening the “Contents” directory, open the “Shared Support”, search for “InstallESD.dmg”
• Click on InstallESD.dmg to put it up on the desktop
Create OS X Mountain Lion Install Drive:
• Start the Disk utility after connecting a USB drive to the Mac
• From the left menu select the USB drive and then select “Erase” option, and choose “Mac OS X Extended (Journaled)” to be the format, then click the corner button of “Erase”
• There will be a option of formatted USB drive on the left side, click on it and then click the “Restore” tab.
• Now, the previously mounted “Mac OS X Install ESD” image should be dragged into the section of “Source”
• Before clicking on “Restore”, the formatted partition should be dragged to the “Destination” section
• You should confirm and allow for the USB Drive to lose all its data and then if asked, enter the Admin password.
A bootable OS X Mountain Lion installer drive will be made by Disk utility out of USB drive with the help of disk image; this will take some time, which depends on the speed of the drive and Mac. Mostly it will take up to 20 to 30 minutes. After finishing you should rebooy the Mac and then bring up the boot menu by holding down the option key. Boot into the OS X Mountain Lion installer by selecting the orange “Mac OS X”. From this point the installation is normally similar. Click on Continue for selecting the destination drive, and then install.