Do you already have the other parent's rights terminated? That is the first step. IF you do and are all ready to go you can try
http://www.rapidlaw.net/Adoption.html They have a full service option, and a forms only option where you do it all yourself. I believe that they email the forms directly to you, or you can have them do all the work for a higher fee.
http://stepparent.adoptionblogs.com/