I don't know if I just got frustrated from not finding any deals or what, but I just dropped $50 on this thing:
http://www.ebay.com/itm/300960762471
Basically, I have a computer that I like to call my 'Legacy Interface' build. It has an AMD K6 266Mhz processor, 152MB of RAM, and a motherboard with an odd mix of options (PS/2 or AT keyboard, PS/2 mouse, ISA slots, PCI slots, headers for COM, LPT, USB, etc), but I've been wanting to run three floppy drives in it for a while. I want to be able to access 1.44MB, 1.2MB, and 360kB disks at the same time, and I want to be able to do 360k disks in a native 360k drive. I'd also like the option of adding a 720k drive in the future, should the need arise (though I might need a new case if I did that, as this case only has one externally accessible 3.5" bay). So I dropped $50 on this thing, which came in on Monday.
Yesterday, I got it set up, and while I discovered the built-in BIOS on the thing either doesn't assign drive letters or is overridden by the BIOS on my motherboard, I can access drives on the 'Internal' connector by setting them up in my motherboard BIOS, and on the 'External' connector by using a driver I found online (SDRIVE.SYS, mentioned on this page, worked fine: http://www.seasip.info/VintagePC/floppies.html ). Now I'm just waiting on new floppy cables to get here so I can have all the drives hooked up at once.
While I feel like $50 is a ridiculous price for an old piece of hardware like this, I'm oddly at ease with having paid that price, considering how much trouble it is to switch out two floppy drives on this machine (I had tried installing a physical switch for that purpose and using DRIVER.SYS to create an alias for the switched drive, but it wouldn't work unless I actually rebooted the computer and changed the drive density in the BIOS).
Anyway, I just kind of felt like explaining it to someone, haha. If anyone has anything relevant or related to add, feel free!
EDIT: By the way, I had also tried simply installing another two-drive controller jumpered in 'secondary' mode, but that controller was either incompatible with SDRIVE.SYS or had some conflict with other hardware in my system, as I was never able to access the drives attached to it without setting the jumper back to 'primary' mode and disabling my motherboard's on-board floppy controller.