Cereal grains are cooked as a general rule, so if you want a cold cereal you must cook it ahead of time. You'll have a hard time imitating "flakes" or sugar coated shapes, but you can make tasty cold cereal at home. And who's to say you can't add miniature marshmallows or chocolate chips?
If you need reasons to make your own cold cereal, how about taste? Health? Cost?
The easiest ingredient for homemade cold cereal is toasted oats. Just spread oats - any kind from steel cut to instant - on a cookie sheet and toast in a slow oven until they're very lightly browned. This can take a couple of hours. Alternately, toast them on the top of the stove in a heavy skillet over medium heat. You'll have to stir them often but they toast much faster.
Puff or pop whole grains by adding a few to a hot skillet and shaking it around or stirring until the grains pop. If grains are old or too dry, they won't pop, but most will. Grains to pop are amaranth, wheat, spelt, barley and brown rice. Sort through the grains first and remove any broken or malformed grains as these won't pop. Popping grains is an art rather than a science!
I've tried other things with various results. Some might work better for you.
Soak wheat or spelt in water over night, then blend it (adding more water if necessary) until you have a pulp. Cook this pulp until it's smooth, then spread very thinly on a cookie sheet or dehydrator trays and let dry. I didn't use heat to dehydrate it, but that might work well. Once it's dry, break it into small pieces for cereal.
Store your cereal in air tight containers so they're handy when you want them.
Once you have a good assortment of cold cereal ready to eat, you may never go back to the over priced, over sugared, over treated chemical concoctions they call breakfast food.