Unfortunately, the only way to get the Walton's Gang outfit for an Xbox 360/Xbox One version of RDR1 is through the GOTY edition. The Undead Nightmare Collection only includes the Undead Nightmare, Legends and Killers and Liars and Cheats packs.
You're right - the GOTY edition isn't sold digitally on the Microsoft Store. You'll have to do what I did and buy the disc version. I bought the GOTY Edition disc through Amazon. It came up to $30, but this is because I live in Canada and video games are more expensive here than they are in the US. You can probably find the GOTY disc for cheaper through other online marketplaces though.
Also be aware that the GOTY disc contains the bugged-out version of Undead Nightmare, with the disappearing heads and all of that. If you still want to play Undead Nightmare without the bugs or glitches, buy the Undead Nightmare Pack or Undead Nightmare Collection digitally through the Microsoft Store, and boot up Undead Nightmare through your digital RDR1 copy (assuming you have RDR1 digitally. If not, you'll only have the glitched out version of Undead Nightmare to play. Sorry!)
