After you’ve revised your manuscript several times, you should have it professionally edited. There are different levels of editing, from basic proofreading to more extensive editing which includes such elements as character development and flow of story. Finding a good editor can be tricky, especially because there are no formal certifications for people in this field. You may want to network with writing contacts, groups, associations and communities to get started in your search. I recommend considering a potential editor’s background, experience and really scrutinizing their website for grammatical errors and so on. Most importantly, I recommend having them edit a sample of your work to see if she is a good match. I regret to say that I can’t offer a site with a listing of editors. I used word of mouth to find editors for my first two books but recently, I discovered a wonderful editor through the site, Reedsy.  A professional edit of novel can cost anywhere from $500 and up, and although pricey, it is essential.