I buy the Just a fold III covers, either 10x21 or 10x23. Either will fit a standard sized hardcover dustjacket - the wider ones are good for thicker books (ie. More pages), as their dustjackets are also wider.
I also get 9’ covers and 12’ covers for smaller hardcovers and oversized hardcovers, respectively - good for some small press books that aren’t standard sized.
The Just a Fold is “adjustable” - you fold the cover lengthwise to fit the book height, along perforations. With a bit of practice they can look very nice, although the paper backing does add some thickness to the book.