Information that may be helpful for obtaining a unique (club) callsign in the US can be found below:
However, a unique callsign for ircDDB may not be needed for your
purpose(??). Below is a cut and pasted note from an article discussing
callsign registration.
Note: Many of us, for callsign routing,
moved on from G2 to using the ircDDB system. But some people did not
like, when using ircDDB for callsign routing etc., to have to also
register at ircddb.net. These people believe each country’s FCC
equivalents have regulations about being licensed, and that is enough,
and in any case, there is no way to stop people from breaking the law
and using someone else’s registered callsign.