It sounds like you are trying to download and install OS X Lion and have purchased it in the past, but you are not seeing it in the Mac App Store.
First, I recommend signing into the Mac App Store with the Apple ID you used to purchase OS X Lion and looking in the Purchases pane of the Mac App Store to see if OS X Lion is listed there:
If you are not seeing OS X Lion in the Purchases pane, I recommend checking to see if it is a hidden purchase using these steps (click on the image to enlarge it):
To unhide your purchases
- Open the Mac App Store.
- Choose Store > Sign In, then enter your Apple ID and password.
- Choose Store > View My Account.
- Click on View Hidden Purchases from the Account Information page.
- From the Hidden Purchases page, locate the app you would like to unhide.
- Click the Unhide button.
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I can't edit my last post somehow, but I found a method for those who have Lion preinstalled, interested to obtain an installer:
It says to just initiate the reinstalling Lion process with the regular Recovery HD. But choose to install Lion on an external drive. When the download is about to finish, be aware. When the screen goes black, remove the external, and you'll have the installer package on the external drive.