Welcome to the forum and the world of immersive audio! When you get the AppleTV 4k box and subscribe to Apple Music, have a look at this page where I organized many of the albums I found on Apple Music in both Dolby Audio (Quad and 5.1) + Dolby Atmos. Immersive Streaming | Added - Hi-Res EditionI’ve got an STR-DH740, which I guess is a little too old for Atmos (but still sounds great to me). It looks like the Apple TV thing is the way to go-I can get one for the same price as a couple of the 5.1 discs I’m after, and I can just cross my fingers those mixes will show up on Apple Music!
Also, I want to put in a plug for Blu-ray and DVD-A/DVD-V's which have plenty of surround and now Atmos mixes on the BD's. These also get reissued, and some of the older surround SACD's have been reissued in either of these hi-res immersive formats too. There is plenty of music to collect that will keep your ears happy for a long time! Enjoy.
Oh, and also some of the OOP items are cropping up on download sites, the most well known is Shop | IAA These tend to be more recent releases, not classic albums.