I don't have any specific recommendations since we didn't do a domestic adoption. I do know you don't have to go through an agency or attorney in your state, although the homestudy should be done in your state. I would make a new post asking for advice about domestic adoption agencies and attorneys, and you'll get some responses from people who know more than I do. Keep in mind that cost isn't the only variable, either. You'll want good service, too, in case you run into a situation with the birthparents or some such.
Good luck!