Almost all sweet corn sold in the marketplace is now grown from modern genetics that have high sugar content in the kernels, so it's hard to find older varieties. One variety that still persists that is not quite a sweet is Peaches and Cream, although it's still an improved variety and may be sweeter than some older varieties. You can find Peaches and Cream seeds through a lot of garden stores, but I'm not sure if there are locations to buy sweet corn that's already been grown and ready to purchase. You may just need to ask some sweet corn producers at a farmers' market if they have any of that variety.