Question: I'm aware there are floppy emulators that are installed to 3.5″ bays where disk images are stored on a flash drive. But what I need now is the opposite: essentially a USB dongle that emulates the floppy drive hardware with a disk inside. Something that older OS installers would recognize.

The need came up when I tried to install Windows XP on a laptop. The only way to load the AHCI drivers is via a floppy drive. I have a USB floppy drive, but no disks at the moment :( and obviously there is no 3.5″ bay to install the typical emulator in a laptop.

I know I can always just buy some floppies, and in fact I have some on the way, but given the reliability of floppies over time, I still feel that what I'm describing would often come in handy.

Does anyone make/sell these?

Answer: 
Seems like the answer is NO. While others here have helpfully suggested workarounds to try, the answer as to whether the piece of hardware I asked about exists in a single piece is NO.
The emulator that you want does exist. You are looking for an emulator that emulates a USB Floppy Drive. So the emulator connects to the computer via a USB cable, and the storage media is a USB Flash Drive.

Do a Google search on "UFA1M44-100" and you will find one model of emulator that emulates a USB Floppy Drive. You can find another company that sells these if you do a Google search on "IntelliRob Systems". Look in the menu on the left side of their pages for "1.44 MB (USB) acts as USB Floppy Drive".

I was trying to find a way to get a USB Flash Drive to act as a floppy without any emulator, but I finally gave up and started to look for a regular USB Floppy Drive. That is when I stumbled upon these USB Floppy Drive Emulators. I've already bought a USB Floppy Drive, but these emulators still interest me, so I may yet buy one. If you do buy one, please let us know how well it works out.