Potential speakers will likely want to know:
For what subject?
And to speak in what city?
And for what purpose?

I'd suggest you contact the Oregon State Federation of Garden Clubs. They will have a roster of various garden speakers who can address a wide variety of gardening subjects. Garden clubs almost always feature a guest speaker at each of their meetings and the topics are wide ranging. Local retail nurseries that offer a lot of gardening classes or seminars may also be a good source for referrals.

It is a good idea to narrow down your preferred topic......most speakers specialize in a particular area of expertise. And understand that most speakers are not free.....expect to pay for their time and travel.

Here is a link that might be useful: Oregon Garden Clubs