They were extremely well-known, but like most fantasy authors who published largely before 1990, they've basically been forgotten by what I'd call the LotR-GoT generation, i.e. Millennials, particularly younger ones. Through no fault of either said Millennials or the authors, note. But if books aren't printed and authors aren't being discussed, well, what are you going to do?
Moorcock is most shocking one that even fantasy literature-involved people under about 45 tend to have close to zero awareness of, or to have heard the name and know nothing else. Which is utterly wild given he was almost as he was hugely influential on fantasy as a whole, and even more influential on fantasy in tabletop games - wargames and fantasy TTRPGs - and in videogame fantasy. Indeed, I think a reasonable case could be mounted that for games in general, Moorcock's influence direct and indirect, is actually significantly more than Tolkien's.
With Leiber, book-wise, I'm not sure much is in print, annoyingly, but Swords and Deviltry is the first book, and any collection that includes that is probably a good bet. Personally the first one I read was The First Book of Lankhmar, which includes all of those and another, but I am very confident that is out-of-print and doesn't appear to have a digital edition.