My favorite maple flavoring is Natural Maple Flavor by BaktoFlavors. It comes in a 1-ounce bottle, or a pack of 3. It contains Water, Propylene Glycol, Natural Flavors, and Citric Acid. It is Gluten free, GMO free, and made in the USA. You can buy in at Amazon.com or directly from BaktoFlavors.com.
I’ve also used Frontier Maple flavoring, which is more widely available in groceries, however it appears to contain GMO soy, so it’s off my list for now.