You can find matching products to the design you want in several places:
1. On the page where you choose designs. If the design has a matching set there will be a link to it there.
2. When you click to see more information on a specific product you will see a row of products with the title "Matching products". This is only displayed if there are matching products available for the product.
3. If you have added a product that has matching products to basket there will be a link to add matching products to it.