Fresh coconut can be found at ethnic markets or health food stores. The stores are also helpful in cutting the coconut open. You want to search for the fresh green or white ones that are already peeled. The cost of fresh coconut varies depending on where you live. I usually pay one dollar per coconut.
If you cannot find fresh I also suggest shelf stable coconut water. Just be sure the only ingredient listed is coconut water. There is no need for them to add Sugar or other stablelizers. The cost of these can vary from two dollars to five.
My personal favorite shelf stable coconut waters are: O.N.E., ZICO, and vita coco. These can be found in most health food stores. You can usually find better prices if you buy online.