How do I download Photoshop CS4 Extended?

I had a copy of PS CS4 Extended on my old computer, but the computer broke. I own the serial, but don't have a disc to reinstall it (downloaded it off the net and activaated it with my serial that came with an academic copy of PS CS4 I bought from the uni computer shop in 2010). So I need to install it on my new comp, but i don't have a copy of the program. Where can I download it from? Is there somewhere official? I don't want to download it off some dodgey virus prone 3rd party site