In Georgia we have a heirarchy of State garden clubs and they put out a newsletter plus keep a list of speakers. If you have such a coalition of garden clubs where you are then tell them you want to be listed as a speaker and give your topics and contact info. Each state has a Master Gardener organization. Lots of counties have their own individual Master Gardener club. Contact the main state organization plus the individual counties. Give your name, topic and contact info to all local nurseries. Lastly, contact any nearby Botanical gardens and ask to be added to their speaker list. Do these things plus word of mouth and generous distribution of business cards and you will be swamped as I am, providing you are a good speaker with interesting topics.

Here is a link that might be useful: Bittersweet Gardens

Look for public gardening events, such as home & garden shows, and ask to be on their speakers list. Most of the commercial events schedule this in November and December. That's the time when I sent out a postcard to those who have booked me before with my list of garden topics for the coming year.