Xbox 360 - Unlockable Bonus Content: Which website can I find it for every game?

I had an Xbox 360 10 years ago. Some games had special editions where codes were included to unlock additional content. I think you could also buy the content online. On which page can I find the bonus content for each game? I only find normal games on the Xbox site.

Mostly only on Ebay, but it is offered to overpriced usurers. Rather look at what this one bonus content belongs to, and compare the prices of the "edition" of the game and the bonus content directly. An example: waffenskin XY is included in the GOTY and Limited Edition, but can be bought directly from individual people, but without a game. Compare the editions with each other and then the price of the individual object. It was the same for me at the time: I have almost all the additional content in Halo 4. At some point I discovered the weapon skin "Arctic", it was a combat rifle with winter camouflage pattern, which you could only get for the first time at Gamestop or Walmart when using Halo 4 Pre-ordered. Then, however, the weapon came out as a package, i.e. The cap rifle, another weapon skin and an armor skin, all included in Halo 4 GOTY (Game Of The Year) Edition. You can get the Halo 4 GOTY for just under 5 euro new. (Note, such games always have to be bought new, the codes are almost always redeemed for used games!) However, I found the camouflage combat rifle at an Australian dealer on Ebay, but he snotty demanded just over 200 euro! Only for this one weapon skin! Defuq?! Therefore: always move up if you are looking for individual bonus content, sometimes whole packages are cheaper.