It is frustrating. Often by the time you hear about a show or event you would like to speak at, the speakers have already been selected. That is because most event organizers do not want to promote an event until their line-up is locked down.
However, according to consultant Bob Simee, there is a way around this. "Speakers often drop out at the last minute, especially if there's travel involved. All the bigger shows have drop-out slots they have to fill. If you're on the spot and they know you, you can walk into that slot."
Make a list of the top shows you would like to speak at, find out who the speaker coordinator is, and start pitching them as a replacement speaker. They will need your short bio, plus a list of specific topics you can speak on. Pitch them now, and then call again with a reminder about a week before the event.