2 Expert Insights

There is no substitute for dynamic human engagements. In other words, getting together with talented people is much better than reading stuff online.

Make your gatherings as interesting and useful as possible. Invite the best speakers you can, and create opportunities for people to meet and get to know others. If you focus on giving participants the best experience possible, your membership will grow as people pass the word about each meeting. Don't rush it, just focus on executing in the best manner possible.

I agree with Bruce. Though there is a lot of useful information on the web, there is no substitute to human interaction.
What you did not say is where are you located and what kind of speakers would your association be interested in. You have to first decide what subject the speakers need to talk about and this would depend on you associations goals. Second, you need to search for speakers that are familiar with the subject and can say something about it that is new and exiting. Having good references would be helpful as well.
Just do not go to a speakers bureau because you will have to pay them a fee and they might not get a better speaker than you might find yourself.
If I knew more about your association I would be able to give a better answer.