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: 
There's no way a USB anything can transparently emulate a floppy drive without a driver being preinstalled. 

The traditional PC floppy drive was an ISA device and appeared on specific I/O ports (0x3F0 to 0x3F6 IIRC). Reading and writing to these ports was how you talked to the floppy drive.

USB peripherals talk to a USB controller, but do not otherwise have a connection to the system bus. So they cannot appear at the x86 I/O addresses where something expecting a traditional floppy would be trying to read/write.

USB keyboards and mice look like PS/2 device to DOS and BIOS by a sleight of hand called "System Management Mode" - unfortunately this is part of BIOS/UEFI firmware and not easily/publicly available to operating systems to customize.

It may be possible for a device or software to hook into BIOS routines that read/write to the floppy, but by the time you get to that prompt in the Windows XP installer, Windows is already running and not using the BIOS to read/write to devices.

Really doing this would at least require a direct connection to the ISA or PCI/PCI-E bus. And many motherboard chipsets already have a Super I/O chip or equivalent that acts as a floppy controller, and already appears in those locations. 

The HxC devices mentioned by @tofro are probably what you want. Search for "cf card floppy emulator."
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.