Looks like Adobe is now just giving away this old version. You can download from their site here and use the serial number that is shown. Caveat: I already have Photoshop so I haven't tried this. But I'm still on CS3 myself and it really does everything I need. There are also some other old versions here, like Acrobat and Photoshop Elements.
http://www.adobe.com/downloads/cs2_downloads/index.html
http://www.adobe.com/downloads/cs2_downloads/index.html