Annemarie Garden's Maker's Market is the best way to go for the money, last year it was $10 for space, you could park right where you are selling, and they do alot of advertising. They pull in a good crowd, and everything is locally made or grown. I would encourage you to sign up for more then one month- give it three or four- I found we had many repeat customers and it was a great starting point for us to build a client base. You can download the application right online, it does get full so sign up ahead of time.
Leonardtown has a similar thing in the works Second Sundays from May to Sept [?] Locally made[/grown stuff only. I don't know who the contact is yet but I can let you know. The Leonardtown Business Assoiciation is running it.
There is a Strawberry festival in May in Chaptico at Christ Church I think, and you can also sell at Earth Day on Leonardtown Square April 22. SMAWL has a big animal fair in May in the Leonardtown Fairgrounds and they look for vendors.
There are also several juried shows around do you want to know about them? I'm not sure what you are selling, they are much more expensive to be in.
Facebook is a great way to promote the places you will be selling, when ever I am working on something new I post pictures and information on my project woman studio page. I try to post something every two days or so and so far it's worked very well.