Has anyone found a 16-18VAC to 5vDC converter? (i.e. Ring power -> USB power)

This is not directly a Ring question, but it came up in my Ring install so I thought I might not be the only one. I replaced an ancient closed circuit video system with Ring, and so far all is well (save for a question about some image blurring in the first 3 seconds of a motion capture, but that is posted elsewhere). The old wiring has one pair going from a 18V AC transformer to the monitor station, and a door bell pair leading from the monitor station to the old doorbell location (now occupied by a Ring 3+). So to power the Ring 3+ all I have to do is connect the doorbell wires to the transformer wires and to the Ring3+ and I'm good to go (no wire wound resistor needed for Ring3+!). Except that I would like to replace the monitor station (now simply removed) with a semi-permanent iPad and so I need USB power at this location. Thus I would like to connect a 18VAC -> 5V DC converter in parallel here to power the iPad, in addition to connecting the 18VAC pair direction to the Ring 3+. So far I have not found a conveniently packaged 18VAC->5VDC, or better yet 18VAC->usb charger box that I could stick into this wall box. So just wondering if anyone else has run into something similar and found a nice solution.