There are several ways to find speakers for your event. You can invite them, for example, or you can put out a Call for Papers. Many conferences use a mix of both.
At TEDxStuttgart, we are constantly looking for good speakers and interesting topics. Some we find ourselves, some are recommended to us, and some suggest themselves via our website or by email; and we found quite a few good speakers via that latter method, so a little self-promotion is not something we frown upon.
That is, if you do it right ...
