Is there a way to buy the stances contained in the Champions Bundle separately? I already have the map pack that’s included since I bought the original DLC pass.
No, the stances are Champions Bundle bonus content and are not available separately.
Also, the War Games Map Pass only covers the first three map packs: Crimson, Majestic, and Castle. It does not cover the Bullseye Pack. It says that on the description in the Xbox Live Marketplace.