You can find Title Update files and DLC Unlockers (specifically the TU3 update). Place the DLC folders in Hdd1\Content\0000000000000000\435707D4\00000002\ . Use Dashlaunch on your RGH to enable contpatch = true (content patch) to unlock the DLC without a license.
Note for Xenia users: DLC support is experimental. Some car packs load, but the Rally expansion currently crashes the emulator when loading the rally map. Even with a perfect ISO, you might hit issues. Here is the frequency chart: Forza Horizon 1 Xbox 360 Iso
Fast forward to today, and the search term is exploding in popularity. Why? Because Microsoft has delisted the game from digital stores due to expired music and car licenses. Physical discs are becoming scarce and expensive. For preservationists, late adopters, and nostalgic veterans, the ISO (the exact bit-for-bit copy of the game disc) has become the holy grail. You can find Title Update files and DLC