Like gabe said you can use ANY corn, you just have to handle it properly. Flaked corn is pre-gelatinized, it is in a sense already "mashed" for you, it's prepared to use. But you can take any corn product and do a cereal mash (google it) grits, corn on the cob, canned corn, creamed corn, polenta, cornmeal, corn starch. You can treat air popped popcorn, corn flakes cereal and even tortilla chips as pre gelatinized, and just toss them in the mash tun, or try a cereal mash and see if there's a difference. Then there's obviously corn sugar and corn syrup (without additives, there's one brand that is pure syrup without vanilla or anything added, (I can't remember the name, and it's hard to find, but there might be one in a health foods store.)
So basically if you find some corn that you feel comfortable using, then you just have to treat it accordingly.