Ohhhhhhhhhh I just found a good site!
It doesn't seem to have everything and the search is a little weird, but check out
www.drinksmixer.com it seems like you have to look for a drink, but then it gives you the ingredients and you can click on the ingredient name and check the carb content.
For example, I just found out that all of the Absolut Vodkas (even the flavored ones) are 0 carbs. Cool!!!
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I'm suddenly not so sure about the accuracy of the site. It says that Tequila Rose is 0 carbs and I have a hard time believeng that what with all the creaminess.