I got the installer from my work's IT service. I needed Indesign CS6 for work (working at home a couple of days a week) so luckily they still had the installer (we're still on Mac OS X.12 Sierra for the time being).
And you're right, you can't download any older versions anymore, even the first CC versions. Just a way of making you upgrade. It's scandalous : my company bought the licences at the time but now you need to be subscribed to the new Adobe CC as well in order to activate your installed version of CS6. It's almost criminal. So basically, you can't use what you already paid for unless you pay them more.
In the end, all Adobe wants is your money.
Looking at Affinity suite, my company as well as myself, to find out what's possible or not. And we're far from being the only ones. Adobe has delusions of grandeur. They'd better watch themselves, as greed could cost them dearly...