ScreenSleeves generally receives the album cover art from whatever music player you're using (Spotify, Apple Music, iTunes and many others).
ScreenSleeves has traditionally had the option to look up album art online where necessary. (In fact, it had the option to *always* look up art online when using Snowtape, because the method SS used was more likely to produce the correct art than Snowtape itself.)
ScreensSleeves has used Gracenote for a few years. GN is a firmly commercial operation but they allowed me (with the actual number of requests capped) to use the service for free. At some point recently that lookup service seems to have unceremoniously stopped. My account doesn't exist any more and there's no longer any information about the lookup / API. (Often the way when you make use of 3rd-party services.) A quick Google shows that software and even hardware that made use of CDDB for CD information no longer works since a certain date.
MusicBrainz is a much more open music database. They allow free non-commercial use. I have discussed Screensleeves with them and come to an arrangement which allows SS to perform the necessary lookups when necessary.
This is a good solution. I have some old and obscure albums that it doesn't have artwork for, and even the odd album that it doesn't even have in the database (I must learn how to contribute to the database). However, the search works beautifully and when the artwork is available it's very good quality.
With this up and running in the development version of Screensleeves, I was impressed with the quality of the artwork and wanted a way to simply perform a lookup and save the cover for my own use in iTunes. (My personal media server uses an old version of MacOS and iTunes, because when something works I like it to remain the way it is, rather than changing at the whim of the maker.)
I've built an interface around the MusicBrainz cover art lookup that I'd written for ScreenSleeves and this is the result.
The download is here (well done for finding it. At this point I won't publish it on the peacockmedia.software main site or anywhere else).
If this interests you, please let me know in the comments or by email. Other apps are available but they do tend to integrate with your iTunes/Music library rather than simply allowing you to save the artwork and do what you like with it.