I have found it really annoying to be late to the Forza party, After playing hours of Forza Horizon via Xbox One X BC (in 4K) I found that I was missing some DLCs and they are delisted.
Some places online are still selling working keys for these DLCs and I will list what I have found so far and whether it works or not.

I am in the U.S. so I have a NA Xbox Live account and Store on my Xbox One. download all items via the Xbox game list and not ingame

Forza Horizon:
1000 Club
Not Available, I will update this if someone finds a place selling codes

VIP Membership
Not Available, I will update this if someone finds a place selling codes
GameStop had it but the link 404'd sometime last year so I assume they are out.

Rally Expansion [if you get an error about waiting until December when trying to use rally, launch the game again]
GameStop https://www.gamestop.com/xbox-360/add-ons/forza-horizon-rally-expansion-pack/116334 | Does not work anymore, order is cancelled within half an hour.
Amazon UK https://www.amazon.co.uk/Forza-Horizon-Rally-Expansion-Download/dp/B01LDPT5QA | Works, Needs a real EU/UK address (Google one) to complete the order.
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Forza Motorsport 5:
VIP Membership
Newegg https://www.newegg.com/Product/Product.aspx?Item=N82E16832596516 | Works
GameStop https://www.gamestop.com/xbox-one/add-ons/forza-motorsport-5-vip-membership/116359 | Have not tried, at worst they cancel the order and never charge you.

Car Pass
Newegg https://www.newegg.com/Product/Product.aspx?Item=N82E16832596515 | Have not tried.
GameStop https://www.gamestop.com/xbox-one/season-passes/forza-motorsport-5-car-pass/116362 | Works

 

[netovsk]

Nice tut. I got F5 through gwg but never played it since my xb1 came bundled with F6. I remember it having a sort of platinum/ultimate/w.e. edition for like $5 or $10 before coming in gwg.

I am in the U.S. so I have a NA Xbox Live account and Store on my Xbox One. download all items via the Xbox game list and not ingame

You probably know this but all you need to do to access other countries stores is to change your xbox region in system settings and restart the console. You can do it as many times as you'd like and comes in handy for accessing game pass/gwg titles not available in your country (although I bet the NA store is the most complete).

 

[driverdis]

You probably know this but all you need to do to access other countries stores is to change your xbox region in system settings and restart the console. You can do it as many times as you'd like and comes in handy for accessing game pass/gwg titles not available in your country (although I bet the NA store is the most complete).

I did this to get the Animelab app since it is on the Australian store, the Xbox One makes switching stores really easy.

As far as I can tell, the Forza dlc codes are region-free so hopefully this guide can help people in other regions without them changing console regions.

 

[Paccc]

You can just change the country-code in the web shop to get free stuff only available in other countries.
(to buy something in a cheaper region you usually need to have a gift-card in that currency bought from some other dealer)

It is super-annoying that things are de-listed without notice,
Forza Horizon 2 Fast and Furious just disappeared and loads of FM5 free dlc became unavailable just weeks after it being GWG...

But I think the best solution would be to add really rare codes to a burner account that you can gameshare with friend later on, perhaps as a trade.
Forza Horizon 2 will be de-listed this autumn so you'd better get those DLC in the next sale (the game itself will likely be GWG)