I don't have a source for a ready-made serial interface, but I did modify an off-the-shelf USB serial adapter to work at 3.3V for another project. I will test it on my RPi and report back.
Basically the steps are:
1) Identify and purchase an FTDI-based USB to serial adapter, such as this one:
(Yes, they all look the same, shot in different coloured plastics. It's often hard to tell what's inside. Could be FTDI, could be Prolific).
2) Cut off the plastic moulding to expose the PCB.
3) Desolder the RS232 level shifter (the one that isn't the FTDI chip), support components and 9-pin connector.
4) Study the datasheet for the FTDI chip on the board. It's probably this one:
http://www.ftdichip.com/Support/Documen ... FT232R.pdf
Look at diagram 6.4 on page 26. What you need to do is hack on the board and cut the 5V connection to VCCIO and add a small wire to connect 3V3OUT to VCCIO. You could install a jumper as indicated in the schematic to make it 5V/3V3 switchable.
4) Identify TxD, RxD and ground pins, and solder on a flying lead with 0.1" receptacles (you can get a housing with three in a row, such as the fan power connector mentioned above.)
5) Plug the receptacles onto the RPi GPIO serial port pins. If you use a 3-position housing then make sure the wires are in the right order, and it's plugged in the right way round.
6) Plug the hacked board into your host PC USB port.
No warranty express or implied. Do this at your own risk.