i also stuggle with low-calorie (and low cost!) non-alcoholic alternatives, especially since i try to avoid caffeine too so no diet coke for me! if only more bars did something as simple as diet lemonade...
i generally go for a small fruit juice like pineapple or cranberry (healthy and not too many calories if its a small) topped up to a pint with soda water - refreshing, fizzy and the soda is free! or blackcurrant.lime cordial with soda is always good, but can be quite sugary.
or, if you know you'll be in the same bar for several drinks, try getting one small glass of wine, and a pint or two of soda/lime and soda, plus a spare glass - and mix them to make several weak but tasty spritzers! you'll feel like you're being good but having a treat at the same time, and only be having a fraction of the calories and alcohol.