Is there a Stack Exchange site for shopping, e.g., men's wear?
I am looking for a good online shop for a turtle neck, American size.
I don't think there's a Stack Exchange site for that. Most SE sites frown on recommendation questions, since they attract spam. Most of the sites are for professionals and enthusiasts looking to learn about a subject, so it's possible that someone might propose a site (on Area 51) about fashion or men's wear, but it's unlikely that they'd start a site where recommendations on where to shop would be on-topic.
No, and it's pretty unlikely. While there's been a few experiments, with product recommendations sites, it's not really something that fits the Q&A model very well.
Website recommendations (specific as in your case or general) aren't something that's happened yet and what a "good online shop" is is kinda ultra subjective and not a great subject for a question.