There are three ways: (1) on the ingredient form choose Supplier Default (and optionally add a Supplier IDs row for that supplier so the cost is derived automatically); (2) in the Ingredients list click the Supplier cell and pick a supplier from the dropdown; (3) in the Inventory Shopping List report click the pencil next to an ingredient, choose a supplier in Change Supplier (a price preview appears when that supplier has a Supplier IDs row) and Save, which also updates the ingredient's cost from that row. Confirming an item individually during a receipt import (or creating a new ingredient from one) sets the receipt's supplier as that ingredient's default supplier; using the bulk Confirm All action only sets the default supplier for newly created ingredients, not for existing ones it updates.