Whilst there are many agencies representing celebrity public speakers, the semi-professional speaker seems to rely on word-of-mouth and some low-key directories of speakers. Like any other business, you can promote yourself, but it may be better to work with other public speakers and jointly promote everyone.
No speaker can cover every topic, event or location, and so competing with each other should not be an issue. But a website directory of participating public speakers and some good public relations would be an effective and cost effective way of promoting everyone’s interests. Run as a co-operative venture would make the costs minimal.