AstroSpeakers.Org have kindly let us publicise their page to help Societies find Speakers for their meetings.
Their website can be found at https://astrospeakers.org/index.html (this will open a new page)
A preview can be seen below
There are other Sources as well
The RAS have the following
https://ras.ac.uk/education-and-careers/for-schools-and-teachers/1834-list-of-school-speakers
https://ras.ac.uk/education-and-careers/outreach-map
Speakernet is has a wide range of topics including Astronomy, will require a search.