Whiskey Sour Mix is very similar to sweet and sour mix. I think the liquor store guys were a little off... there is a Whiskey sour (drink) and a whiskey sour mix made especially to use in that drink.

Since you already have a recipe for this, but just need to find Whiskey Sour Mix, I've added the link.

If you like Midori, try this one:

MELON BALL
1 part Midori
1 part Vodka
1 part Orange Juice
1 part Pineapple Juice
(its a little strong, so if you want to add more juice to taste, you're welcome to do so!)

ENJOY!