Citric acid can be found in most grocery stores in the canning section but alot of those have preservatives that are a no go. Best bet is go to a health food store, 99% of the time any health food store will just have pure "citric acid". It looks.like salt and.is suppossed to be a safe alternative to salt (but I dunno if I buy that, smells too citrusy to be a salt replacement if you ask me). Then dissolve about 1/4 teaspoon in about 10ml PG to get a 10% mixture. Youll have to warm.it and shake it a bunch to get it to dissolve though. When you have that done, just add 1 drop p/ml of finished solution and there you go (i.e. if you have 30ml of juice use 30 drops of citric acid solution). Alotta professional mixers use citric acid to get their fruit.flavors to "pop", including Bombie's famous Nana cream. Hope that helps and if thats not the secret ingredient it'll still help out alot with your more tart fruit blends
Edit: btw citric acid is also very cheap, a 5$ bottle for me is enough to make over 5000ml of citric acid solution