Apple now provides the installer as a free download from their official support site, a shift from when it used to cost $20. Official Download Link
Websites like SoftNu or other software archives might host the installesd.dmg file for download. However, exercise caution and ensure the integrity and safety of the file by checking its hash and reading user reviews.
To download and prepare the Mac OS X 10.7 Lion InstallESD.dmg
Right-click and select Show Package Contents . Navigate to Contents/SharedSupport/ to find InstallESD.dmg . System Requirements How to Download Mac OS X Lion and Mountain Lion for Free!