This may be short notice because usually the more well-known authors book their events way in advance. However, you might want to try nearby universities–maybe a professor from one of their depts. has written a book and could do it. Look at the websites for some of the publishing companies (you might do better with the independents) and see if they have listings of available authors. Try a search for authors in the fields you’re interested in and see if they have a list of guest authors. Definitely contact the bookstores (big box ones as well as independents) and see if they know of anything. If there is a lesser known author that you know his/her work, you could contact them (check their website or contact their agent) and see about availability.

I’m really sorry, I know this wasn’t much help. In a previous job I had to help my boss find speakers for dinners and sometimes the wait list was two years or even more, depending on popularity.

I don’t think anyone carries around that kind of information in their back-pocket. You’re going to have to do your own detective work because we’re certainly not going to do it for you.