I don't quite know where to begin with this one because there are so many different types of SEO services. Are you just looking for some extra help or for someone to actually manage and do all of your SEO work? SEO agencys are the best place to go, they carge quite a lot depending on which keywords you want to target but at least you know they will do the job right. It takes a lot of pre planning and work. One thing I can say is avoid any companies that ''guarantee'' page one spots because thats a red flag. Im not saying it can't be done but there are no guarantee's with Google.