I am disappointed no one else has chimed in here to add a suggestion for you. I think that going to more ethnic recipes is the answer to your question. I would point to Iranian/Persian foods which emphasize sweet and sour in most of their recipes. Iranians love sour and use a variety of sources that are unexpected to western tastes. Unripened grape juice, unripened peaches, sour cherry juice and dried sour cherries, pomegranate juice, sumac, dried sour plums (alu bokhara), dried and powdered Persian limes, and my favorite barberries (known as zereshk) are all sources you can go to for a wonderful sour taste. For a sweet and sour balance I would recommend stuffing a fish with a mixture of barberries and herbs and finishing off the pan with a pomegranate glaze. There are numerous recipes online for Iranian foods and you can adjust from there for a lower calorie meal. You may have to seek out an Iranian foods import store for these items – and there are many now that offer these foods to be ordered online.