From any page in the website, click ‘Yarns’ in the top menu. Roll your mouse over ‘Yarn Weight’, and on the right click on the weight you want out of the list.
If you want to select from a few types of yarn, go to all knitting yarns.
To filter by weight, on the left of the page under ‘Refine by’, check the box by the weights you want to use, or search for it in the search box. You can select as many as you want.
If you’re on mobile, select filters at the top of the page and then choose from the options presented.