The short answer: Unless you are a Belmont student, there are none. Only students will have a shot at whatever tickets come open for them.
Here's Belmont's answer to the same question:
Belmont University is thrilled to have been selected as the site for the 2008 Presidential Town Hall Debate. It is a privilege for us to host this historic event.
The debates are primarily intended for the television viewing audience. Tickets are not available to the general public. Belmont students will be given top priority for any tickets that may be made available to Belmont.
The Belmont debate is the only official Town Hall Debate. Participants asking questions of the candidates will be chosen by the Gallup Organization. Belmont will have no control of the selection of participants.