SELECT *
FROM
(SELECT DealerCode FROM Sales GROUP BY DealerCode) m
CROSS JOIN Forecast fc
LEFT JOIN [Sales] s on s.DealerCode = m.DealerCode AND s.ForecastID = fc.ForecastId
ORDER BY m.DealerCode
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden