It’s available on the add-on sale now, but gives you an error every time you try to buy it. This is a known issue, I was unable to buy it from the Xbox store or the browser, nothing available in the downloads section of the Oblivion game menu as well, says “no new content available”.
They are getting ready to shut down the 360 store soon, but maybe it is still purchasable through the 360 store using the browser. However, I moved from US to Europe and my 360 account is stuck in a region lock with many purchased games, they may become inaccessible if I switch back to US. Any help is greatly appreciated!