Back when I first started with Calibre, I documented my paperback collection.
Many were so old (cover price 35cents

), the did not even have a (printed) ISBN.
I would type in the Title and Author, slap the book on my flatbed scanner (HP 4C) and use that output (scaled 50%) as the cover when a search did not find one (or it was a newer edition).
Now-a-days, we have phone cameras with decent DPI, So why not take a picture, Calibre has a crop tool built into the cover section of the MDE, so all yo need as Light and a way to keep the camera square with the book.