Where to find Mac OS Catalina .dmg file?

Hi! I'm trying to find the .dmg file for Catalina (10.15.7). I'm running a late 2012 iMac and Catalina is as far as I can go. It was running nicely, but I had to reset the system to create a fusion drive. I thought that Catalina would simply install itself in the new partition, but I obviously thought wrong and got downgraded back to Mountain Lion.

I found a .dmg file for Catalina from some random source, but Mountain Lion can't upgrade straight to Catalina. I got the .dmg file for Sierra with the following link:

How to download and install macOS - Apple Support

I installed Sierra and hoped to be able install Catalina with the file I had, but it was somehow a damaged file. I tried to download Catalina through the App Store, but to no avail. There is a page for Catalina on the App Store, but no download button available. I'm running the App store in German, and where it's supposed to say "Download" or "Install" or what not, it just says "Öffnen", but it's just a cosmetic element with no function.

Is there a functioning .dmg file for Catalina anywhere? I know it's already quite old, but most all of the apps I need work well with it and it would be a shame to have to get a different device, as I'm quite happy with the iMac itself. Thanks in advance.