Our family library has a bunch of them, since my mom is a cat lady for old books and will buy most anything that looks lonesome. Ebay is the best source. Etsy has some. Facebook marketplace is good too. Shipping rates are horrific now :(
I've got three sets of Britannica at my house, including an 11th edition that's been in the family since it was new. Teeny-tiny print on onionskin (India paper) pages, like nothing made today. They issued a 3-vol supplement after WW1, and after that it was all downhill. The "new and improved" EB, called "Britannica 3" had a bigger selection of bindings than Barbie has wardrobe, but IMO did not live up to the hype. (I was homeschooled so I'm not just being a snob, I read these books.)
World Book is great for educational purposes, as is the underrated Book of Knowledge circa 1950-60's.
Digital versions will never beat tactile! If you doubt it you have not touched a Trans-vision overlay page in World Books. (Human anatomy, Frog, and the best ever, the Ocean.)
I had a late-60’s set of World Book encyclopedias and agree. They were perfect for young students and the overlays you describe were awesome.