There are a lot of good foods out on the market, Hikari is a good name for a fodd available at most Pet Store outlets. I use BlueRidge mountain 50%, Hikari 25%, and aquadine with spirulina 25% mixed for growth along with all kinds of fruits and veggies. I'm not looking for big growth for mine but good balanced nutrition. I want to see good bodied healthy fish, not fat bellied swollen fish that grow huge but don't have good shape. IMO, I would rather a fish take five years to reach 24" and be well shaped than a three year old fish that is 24" and over weight.
Good quality food comes at a price. cheap foods can be found that grow fish for $2-3 per pound, medium quality food can run in the range of $3-$6 a pound. Some of then bests food can cost $5 -$15 a pound. If your going to use some of them it's best to buy in bulk but remember that the better the quality of food the shorter the shelf life and that's pretty much fact. Also, and I don't really know why but Koi food is kind of opposite from Dog food. Dog food the higher the quality the less filler and therefore less waste, Koi food is the opposite the better the quality of food and the better the growth rate from the food the more waste the fish will emit. So be sure and have your filters at max potential when trying to use good quality food for growth.
Too much of a good thing can lead to filtering issues and without good filtration your not going to get the results your after unless the water quality is there to match.