Get Sony A7R IIIat B&H Photo.
There were/are issues with Sony’s updater for the A7R III with macOS High Sierra. Sony claims to have fixed these issues as of today:
If the System Software Updater does not support macOS 10.13 (High Sierra), you need to download "DriverLoader_1013" from this website and run it on your computer first. First, go back to the previous page to download the System Software Updater, then follow the below process to perform the update.
It’s a mess IMO. No user should have to jump through these hoops.
After 3 years, Sony persists in this awful policy of requiring administrator level access on the computer to update the camera. Given Sony’s checkered history and massive security breach not that long ago, one might be wise to always wait a week or two just in case malware slips in, since the Sony updater is a potential root kit exploit.
I dislike this risk so much that I go to the trouble of cloning the startup drive, booting off it on a spare laptop, then running the update, then wiping out (formatting) the temporary startup drive. It’s quite the nuisance, but worth the trouble for the day Sony is compromised again.
Nikon does it right: download one file, copy it to a card, and the camera does the rest. Ditto for Canon and some other brands. In this day and age of malware and security threats, involving a computer as Sony does is a fundamentally unsound decision.