AOR
sells the complete interface
(complete with plug and bundled with a software CD) or the plug alone,
as a spare part. Cheaper non AOR alternatives can be found on the
market (
here for example), but
you'll still need the connector.
Projects are available on the Net to home-build the interface and a
search for "level converter interface" will bring up several links.
The connector has a cheaper equivalent in a HP Jornada spare part
(
here for a nice and
complete project).