The best thing to do is make your own mix. If you do not want to make a sugar
syrup that is an important part of the mix than use a very fine powdered
sugar.
http://www.webtender.com/db/ingred/211
Sour mix, also known as sweet and sour mix or bar mix, is a mixer made of
lemon or lime juice and sugar syrup. It is an important part of many cocktails.
To make your own, mix one part heavy sugar syrup (3 parts sugar, 2 parts
water) with one part lemon juice, and add 2 egg whites per liter of mix. The
egg whites are optional, but will make the drinks slightly foamy. You may want
to adjust the sugar/juice ratio to give the mix the right balance of sweetness
and tartness.