mara's homemade, on e.6th btw 1st and 2nd avenues. i haven't been there in quite a while, so i can't vouch for the food, but they have a strong, sweet iced tea.
brother jimmy's has it- probably not as good as the authentic southern kind, but not bad at all. there is also an arizona iced tea version sold at delis/grocery stores.. haven't tried it yet, but curious.