I've been asked this kind of question before sometime ago by several people. Seems cotton candy is a favorite in kids flavorings, but hard to find.
I'm not exactly sure what flavorings are used in making cotton candy flavoring, as I've researched this many times before and found nothing.
I find lots of recipes for homemade cotton candy syrups, but all they state is "add flavoring" but NEVER stipulate WHAT flavoring it is they use.
If I were to guess at making a homemade cotton candy flavored syrup for flavoring desserts, I would probably use vanilla and bubble gum flavoring, with a tiny, tiny hint of orange extract. But cotton candy tastes different to different people, so I would assume it's "per taste".
I'm almost certain if you use a "light" amount of bubble gum flavoring, nobody will know the difference, as most bubble gum flavorings taste like a heavy cotton candy flavor to me.
And Norcalbaker is correct about adding already flavored candies to something. Depending on what those candies are and made from, will depend whether they can work or not. But, 90% of the time, they just make it taste weird.
And I know for a fact, because I've tested this out many times with different types of candies over the years, as I like to experiment.
I think the best candies to use, depending on what you are making, is something you can melt into a sauce or syrup, like Skittles. Their flavorings seem to stay as they are with little going wrong when mixing them with cake batter or frosting.
But unless you have time for experimenting, I don't suggest trying.
My best advice for you is if you can't find the flavoring you need in time, then I would look for Cotton Candy flavored Jello. Now this, you CAN mix with cake batter and frostings. One small package in one box cake mix usually does the trick (add about 1/4 cup more liquid though), as the gelatin will soak up a lot of the moisture.
If you are looking to flavor the frosting, then I would suggest looking for Cotton Candy flavored ice cream topping syrup. This works pretty well with frostings.
If all else fails, make a nice vanilla cake and decorate it with the cotton candy flavored candies.
Hope that helps.