Not sure if they carry it anymore but yogurt marketed as "European style" often has buttermilk: https://oatmel.com/products/European_Style_Smooth__Creamy_Plain_

Not sure if they carry it anymore but yogurt marketed as "European style" often has buttermilk: https://oatmel.com/products/European_Style_Smooth__Creamy_Plain_Whole_Milk_Yogurt_Trader_Joes_c1e2eb5ac https://oatmel.com/products/European_Style_Smooth__Creamy_Pl... But not sure I would consider fermented milk to be an "adulterant" in a different fermented milk product...