I really like sour flavors in gummy candies.
The official way to make sour gummies is to add Ascorbic Acid Powder. Other powders that will add sour are called Citric Acid or Vitamin C powder. You can buy it in a cooking store, a health food store that sells supplements, probably some pharmacies and I think it's even available at some bulk food stores.
Sometimes it is incredibly expensive, especially if it is from some "Life Extension" or "Body Cleansing" type of vitamin company.
I've never bothered with buying the powder. Ascorbic acid is Vitamin C. I just grind up a Vitamin C tablet with my mortar and pestle. The cheaper the vitamin C the better the flavor I think. Rose hips Vitamin C is not as sour, and has some kind of non sour flavor. I'm not really sure how to describe it.
I'm not sure what the chewables, or timed released Vitamin C's would do, or how they would taste. The added ingredients in those vitamins might not like being heated, and they might cloud up your candies. Just stick to plain cheap generic Vitamin C if you are going to grind your own. For pure sour pucker power, the cheaper the better.
I add the powder of one 500mg Vitamin C to a batch of gummies. That will change the flavor so it doesn't taste so much like a Jello Flavor. I LOVE 3 Vitamin C's - that's a great candy flavor to me. It's one of those things you just have to do by taste. You might like more, or less, than I do.
The easiest way to sour up gummies, and to also give them more concentrated flavor is to add unsweetened Kool-Aid powder! There are a lot of flavor experiments you can try by mixing different flavored gelatin with different flavors of Kool-Aid. They don't come out very clear though.
There are all the other kind of powdered drink mixes. I don't know how they would react if you stirred them into the hot gummy syrup, but I'm sure they would be fine if you rolled the candies in them when you were finished. A good lemonade powder mix would probably taste pretty good.
After it is melted and almost ready to go into the mold, stir in the Vitamin C, then let the syrup sit in the bath for a couple of minutes.
This info applies to candy making in general: When making candy, you will almost always add the flavorings right at the end. The reason is, sometimes flavorings can change flavor or breakdown completely when they are heated. When you are making candy, you will usually cook your candy first, then stir in flavorings right after you take it off the stove.
Have fun and experiment.