Check out lopi wool. That seems to be a high grade of wool Icewear has that type of wool

I just bought my son one so I have been researching.

I think you should go by how you wear clothes. Baggy or snug? Are you wearing it over or under clothes?

Merino wool or other wool?

If you are buying it to wear as outerwear w layers under, go larger.

If you are buying to wear under a coat etc go smaller.

I like sweaters like this a little big. Personal preference.

Amazon is good if the seller is. Aran is a good brand. Machine loomed but popular

Icewear is another good brand.

LLBean has washable ones that seem popular.

Etsy has handmade ones. Hth.