Etsy is the world's biggest website for people to sell things that they've made themselves (such as jewellery, clothing, handbags, etc). So that might be a good place to start:
https://www.etsy.com/uk/search/clothing/womens-clothing/sweaters?q=hand+knitted
However simply having a word with the person who runs your local wool shop might be an even better idea. They'll probably know several people who accept commissions to knit sweaters. So you can choose your own pattern and wool from the shop and simply let them knit
exactly what you're looking for.