If Stiffy Kola Kubez is unavailable, a good substitution would be any cola-flavored liqueur or a kola syrup if you're looking for a non-alcoholic version. It adds a similar cola nut flavor to the cocktail. In absence of these, caramel syrup mixed with a dash of bitters can mimic the complex, sweet-yet-spicy flavor profile of Stiffy Kola Kubez, though the taste may slightly differ from the original recipe.