Photoshop Elements 7

There is actually a download link for Photoshop Elements 7 buried in the comments on the Prodesigntools site. 

Try here (Windows only): https://prodesigntools.com/direct-download-links-for-lightroom-3-and-photoshop-elements-8.html/comment-page-1#comment-2174

I think your only option is, as @Mylenium says, to buy used CD from a trusted source and reuse your own serial number.

Unfortunately, the only site I know of that lets you download guaranteed uncracked and virus-free installation files, Prodesign Tools, doesn't have a link for Photoshop Elements 7. It only goes as far back as Photoshop Elements 8.

Beware of anyone trying to contact you through a direct message, saying that they are an Adobe employee and can help you get the installables. They are scammers and if you're lucky, the only thing you'll lose is a bit of money. If someone does contact you, please report them to phishing@adobe.com.

Not from Adobe, obviously. I'm sure you can find it somewhere on the web or buy a used CD from someoen on eBay. Since you have your own serial, all you need is the program files, so this should be a safe bet.

Mylenium