Where can I download Lion

http://www.apple.com/osx/

At the top right, you'll see a blue button that says "Upgrade Now" That will launch iTunes and you can complete your transaction there.

If you already purchased it, you can just call Apple online Store and they'll give you a code to download it again. If you haven't purchased it before, you might be out of luck. Apple seemed to have really phased it out and hasn't made it available online for quite some time. They were selling USB drives for a while longer, but those are no longer available as well. Some people have commented that they were able to call the Apple online store and purchase it over the phone, but those reports are a few months old.

Best of luck.