A while back, I bought a USB to IDE adapter for less than 20 bucks. It came with a power brick already wired for an HD or CD/DVD drive. It would just need an adapter to power a 3.5" floppy drive. If you could find one of those, you could use the power supply and set aside the USB part "for later".
If you really only need the 5V, you should be able to find a wall wart rated at 5Vdc. If not, get one at 9V. Rectify it to DC if you can only find an AC one. In either case, use a 7805 on a heatsink to regulate it to 5V.
Try this transformer
