Here is the link for your options for Brodart Mylar fold-on, archival book covers.
There are videos on YouTube uploaded by Brodart. I don't recommend everything they have in their instruction videos from an aesthetic and archival perspective for my personal library, but I do like the instructions on cutting mylar rolls, using their mylar pre-sized covers (both paper-lined and not), and how to fold them and apply them.
Let me know if you have any questions in the comments here!
Here is an example of a mylar, folded cover that I have on one of my vintage books that a friend gave to me years ago.