If you want to get Atmos from the most possible services, Apple TV 4K ( $120 at Best Buy) is your best bet. The Apple TV 4K offers the best selection of Atmos streaming options.
Sure, you could invest in standard and 4K Blu-ray discs, many of which have Atmos soundtracks, but streaming is the future of home video.
You also need Atmos TV shows and movies to power it.
If you decide you want it, your biggest investment will be the extra money for an Atmos-capable sound bar or an AV receiver with a full complement of surround speakers. If you're looking for the best home theater experience, you've probably considered Dolby Atmos audio, the most cutting-edge "object-based" surround sound format available. All of these 4K HDR streamers support Atmos, but not on every app.