You can use the Trademark ID Manual for this purpose. A link is provided below:
https://tmidm.uspto.gov/id-master-list-public.html
Under "ID Master List" just enter a category of goods/services (e.g. "Shirts") into the search field, and when you find the relevant category, its class will be designated as the three-digit number under the "Class" column.
https://tmidm.uspto.gov/id-master-list-public.html
Under "ID Master List" just enter a category of goods/services (e.g. "Shirts") into the search field, and when you find the relevant category, its class will be designated as the three-digit number under the "Class" column.