There are several methods that your school could use to provide uniform. You may need to contact your school directly if you are unsure how they are planning to make this available.
Your school may provide a shop where you can purchase any of their merchandise or ticket items. If available, this will usually be advertised on their school website, providing a link to the shop page to make purchases. This has the added benefit of allowing all customers to purchase items listed, not just Parent Account holders.
Uniform payment item
Schools may choose to add individual uniform items to the available payment items list that you can access by selecting the 'All payment items' button on your home page. They may also provide bundle options.
External uniform company
If your school are using an external provider to supply uniform, you will need to contact the school or check out their school website to find out how to purchase it.