Obtainable from Applications prior to installation when downloaded from App Store.
To create bootable USB flash drive: Open Disk Utility, select the drive. Choose Partition tab, select 1 Partition from Partition Layout. Click Options, choose GUID Partition Table, then OK and Apply.
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Reboot while holding down the option key. Select the USB flash drive. For clean install, select Disk Utility. Erase the / partition. After that, select Reinstall OS X.