First, you must create inventory locations and assign inventory items to them. See Use inventory locations | Manager. In general, an inventory location field will then appear after you select an inventory item on a sales or purchase form (receipt, payment, or invoice).
But you can only choose one inventory location per transaction. If you have line items to be sold from or purchased for separate inventory locations, you must enter separate transactions for each location.
Also read the Guide about inventory transfers: Transfer inventory between locations | Manager.