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 =...