upgrade to catalina

Hi - I have a late 2013 iMac 27" with 32GB RAM & Fusion drive 1GB (but all files are stored on a dropbox cloud server and an external HDD.

Currently running High Sierra as don't like updating to new OS when they come out due to previous issues in doing this and as this is a work machine do not need the unnecessary down time.

I now can't install the latest Adobe CC software as it requires OS X 10.15 minimum - which I have found my computer can run (looks like its last supported OS though)

But I can not find anywhere on the Apple store to download this OS only its updates :0(

Can anyone supply a link to download this version of Mac OS X please.