As of my latest knowledge, Forza Horizon (the first game in the series) is no longer available for direct digital purchase on the Xbox 360 Store. The store for Xbox 360 was officially closed on July 29, 2024.
If you are looking for a Horizon game still available for digital purchase on modern stores, consider Forza Horizon 2 (Xbox One version, not 360), Forza Horizon 3 , 4 , or 5 . horizon xbox 360 download