They do make a add in card like you are looking for.... I have to ask why would you want a floppy disk in a modern computer?
Because computers aren't as modern as you think. DOS is still very helpful when troubleshooting, and for those with XP a Floppy drive is a requirement to load a driver when one does repair. If those modern computers never crashed or never required repair, then a Floppy Drive wouldn't be needed. I guarantee your out of luck loading XP (for one) on a new motherboard which requires a Sata Driver or an add an ATA controller driver, if one needed one. Also some Diagnostic CD's and Window Repair CD's require all too often additional drivers, which can only be loaded thru a floppy drive.