I don't think you can purchase McDonalds sweet and sour sauce, but you can certainly purchase it in any grocery store in the Asian foods aisle. Sweet and sour sauce is like ketchup - there may be a lot of different brands, but for the most part, they are all the same.
Currently, it is not possible to order McDonald's sweet and sour sauce online. There is no hotline or delivery available though you can purchase it at any grocery store in the Asian foods aisle. You can also check other related area as they are the same.