I have used the Fanart site. The Album art exchange requires a registration, it seems. All of their art is watermarked in a google search. A couple of sites I use besides google image search is
https://www.covermytunes.com/ and I also look for an album page on
Pandora. The art work there is normally around 1080x1080. Shift+Right click on the image and open it in a
new tab to check it out and save from there. Hovering your cursor on the tab should give you the image size. Also, when I am not having any luck with finding a good picture, I will search Amazon Japan (jazz usually) or Amazon UK (good for blues titles). I can sometimes find a quality image that way.