SACD is a niche product, at best, and probably not a good idea for new investment. If you are looking for high-resolution recordings (higher than CD), the current alternatives are PCM (24/96 is the most common high-res bitrate in PCM) and, to a lesser degree, DSD.
You can buy (download) high-res PCM files in multiple forms, including AIFF and WAV (uncompressed) and ALAC and FLAC (losslessly compressed, for smaller file sizes with no degradation in sound quality). To play them back, you can output them in digital form (from your PC/Mac for example) and process them via a standalone DAC (digital to audio converter), or use the DAC built into your computer. Generally, you can obtain higher sound quality with a good outboard DAC (vs the one in your computer).
MQA is another approach to delivering high-res audio, using smaller file sizes. It is a nascent technology, and there are some good threads here on AK about it.
If you go with an outboard DAC, make sure it can accommodate the inputs you need (eg USB, coax, optical, etc), can process the bitrates of your recordings (some can only handle 24/96, for example), and can handle the format(s) you will be using. In my opinion, you must spend a minimum of $250-300 to get a DAC materially better than what you have in your Mac. Others, of course, may differ.
Keep in mind that one of the "secrets" in the world of high-res is that the difference in sound quality is often due not to format or bitrate, but rather the quality and care taken in the engineering of the recording (including remastering which may be done for the high-res release).