"Nobody makes motherboards with 5¼″ floppy drive connectors!"
To my knowledge (and at a former workplace I had the privilege to use an original IBM AT), nobody ever did. The motherboard had one floppy connector for both 3.5" and 5.25" floppies. The cable did the adaptation (to the edge connector for the 5.25" flopppies). Probably you could use an external 3.5" USB Floppy driver and connect the 5.25" drive to it using the proper cable.
If the problem is just to get data off some disks, look for "Vintage Computer Festival" people and "retro computing" mailing lists. Some computer museums will also be happy to lend a hand. (I donated my old hardware (including a streamer, and both types of floppy drives) to a local one.)