Round Electrical Box Cover, 4 Inch Metal Junction Box Covers with 1/2 Inch Knockout, Flat Outlet Box Cover for 4 inch Round Box and Octagonal Box（6pcs）

Personally, I would not install new fixtures until the electrical box for each sconce is located (appears to be in the wall somewhere). If there’s any upside here, it’s that the wiring attached to the old sconces looks to be factory, and as such, it is probably not more than another foot or two inside the wall. The bad news is that any electrical junction box has to remain accessible (i.e., not behind drywall), for reasons such as the situation you’re currently in. You could purchase a box for each sconce like the following: https://www.amazon.com/ARLINGTON-FSC405R-Sconce-Box-Diameter/dp/B07DMQRWS2/ Mount that new box to the drywall where you want the new sconce to go, and run a piece of 14/2 NM-B / Romex between the new junction box and the old junction box. But you would need to open up the drywall over the old / existing junction box, and keep it accessible by using some sort of cover plate, depending on the size and shape of the box: https://www.amazon.com/gp/aw/d/B0F8HTZWX8/ Or https://www.amazon.com/gp/aw/d/B0F8HG8DQL/ Of course, that’s assuming the person who installed the sconces originally did anything else to code, which they may not have. If those fixture wires are connected to the dwelling’s electrical system by just hanging inside the drywall, and not inside any sort of approved junction box, I would be wondering what other surprised are waiting for me in my walls.