Welcome to the Apple Support Communities
As you have Mac OS X 10.6.8 on the iMac, just open the App Store, go to the Purchases tab and download OS X Lion or Mountain Lion. If you don't see it there, check if it's hidden and unhide it > http://support.apple.com/kb/HT4928
Another option is to redownload the installer on the MacBook, and copy the installer to an external drive
Lion does not appear in the App Store. If you want to purchase it, call Apple using the phone number in the Apple Online Store and tell them you want to purchase Lion. The cost is approximately $20 USD.
Apple will email a download code to you, which you will subsequently "redeem" in the App Store to download Lion.