If a supplier supplies a stock item, you can add them to that item. You can then add the price you get the item for from the supplier as well as any discounts and lead times. This video describes the options available. A step by step guide can be found below the video.
Steps
Open the stock item record.
Go to the Suppliers tab.
Enter Edit mode.
Use the New button to open the supplier lookup and select the required supplier.
In the grid, enter the supplier's Buy Price.
Enter the supplier's reference for the product, if required.
Enter any discount percentage offered by the supplier for this product (optional).
Enter a default re-order quantity.
Mark the supplier as preferred, if necessary. If you have only one supplier for the product, you must mark them as the preferred supplier.