Today, these files are essentially abandonware in legal gray areas. Microsoft no longer sells them. You cannot find them on the Xbox One or Series X storefronts. But if you own an Xbox 360, the hunt is still on.
Note: The Xbox 360 storefront is closed for purchases, but the CDN (Content Delivery Network) servers If you bought it ten years ago, you can re-download it today. forza horizon 1 dlc download