stock that you keep on hand to sell or make something to sell is typically kept as inventory

check your company settings to see if this is in your version of QBO

gear>settings>company settings
sales>products & services
and turn on qty tracking (inventory)
and
in company settings>expenses>bills & expenses turn on the items table and purchase orders. 

Then when you enter an expense or bill, about half way down the left side is a title "item Details" click that and a table opens where you can select the item, qty and item total cost for purchasing inventory

and see the pic for a new inventory item set up