It's a pretty cheesy way to do it, but you can unlock this with 5 minutes or so of work simply by creating a second profile and using that to purchase an item listed on your main profile. All you need is one of the free 48 hour trial Live access codes that come with a lot of games.
From the dashboard, create a new profile (or you can use an existing HDD-only profile) then click the option to join Xbox Live. You need not worry about the email address that you provide - you don't have to access it to validate the account or anything like that. Enter the code from the 48 hour code and you're done.
If the profile is new, you will have to go through the intro and the first race at Swiss Alps before you can start career mode, which you'll need to do in order to complete the first career race and generate some credits (since the minimum storefront sale price is IIRC 1000 credits).
Armed with your winnings, go to Community and to the Storefront option. You can use gamertag as a search field so just search for your main profile GT and purchase whatever you have listed for sale on the main tag's storefront.
It's easier to just get a friend to buy something from you but this may assist if you don't know anyone else who is playing Forza.