Hi mrgrace :waving_hand:
Thanks for joining us here on the Community.
You can enter the Cost of Sales as the expense account when creating the new stock item (Sales > Products & Services > New > Stock) - the CoS account is usually set here by default.
When purchasing this item, you can then create an expense transaction (+ New > Expense) and select the product on the 'Item details' line.
Get back to us below if you have any Q's!