If I understand what you want correctly, you need to make the BranchId filter a part of the outer join.
SELECT
dbo.Product.Id,
dbo.ProductStock.AvgCostPrice
FROM
dbo.Product
LEFT OUTER JOIN dbo.ProductStock
ON dbo.Product.Id = dbo.ProductStock.ProductId
AND dbo.ProductStock.BranchId = 13
WHERE
dbo.Product.Family =...