Possible sources (use at your own risk):
If you know someone who purchased the DLC before 2016, they can still download it from their Purchase History on an Xbox 360 console. Physical Bundles: Look for the Forza Horizon: Limited Collector's Edition
Find a of the original game to start your collection.
If found on grey-market or auction sites, prices can range from $30 to $100+ due to extreme scarcity.
