Have a look at http://www.ftdichip.com/Products/Cables/USBTTLSerial.htm
Depending on your IOREF setting you'd want either the TTL-232R-3V3-AJ or the TTL-232R-5V-AJ.
Personally I went for the TTL-232RG-VIP-WE because I tend to generate the voltage myself
and I never have to worry about voltage mismatch with this one.