Sadly, as the others have found out, there does not seem to be a simple, ready-to-buy, plug-and-play external USB 5.25" floppy drive (just not enough demand probably).
As mentioned, there are some controllers that can be purchased. There is also a PCI expansion card that is geared primarily towards Amiga users, but is Windows compatible. It can even write disks, but at £90, it’s pretty pricey. There are some discussions by people on kludging something together at home, potentially at significantly less cost than either option.
There is a discussion thread by some Mac users on how to make one out of an existing 3.5" USB drive. Another extended USENET thread has a similar discussion. Unfortunately neither one really and concretely confirmed success. However, the attempts and discussions provide some useful information for anyone who wants to try their own hands at it. Some points of particular note include
- The need for an adapter to convert from the 5.25" drive’s edge-connector to the pin connector used by 3.5" drives
- The need for an external power supply since USB cannot provide the 12v needed by the 5.25" drive