At any place like Home Depot or Lowes. Milorganite is one you should be looking into.

Otherwise an animal feed store and get Soybean meal.

Or alfalfa pellets (rabbit chow). Here is a picture of mrmumbles lawn back in 2011. He took the picture in June, about a month after tossing a handful of alfalfa pellets into his zoysia lawn.

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You can clearly see the improved color, density, and growth.
Alfalfa pellets are available at any feed store. Cost should be around $12 for a brown, 50-pound bag. Cost is a dollar more for 10 pounds less if it says Purina or anything else on it.

Application rate is 20 pounds per 1,000 square feet for most all organic fertilizers. Milorganite is an exception. Read the bag. If you apply too much, no problem. A guy on another forum experimented with applying 50 pounds per 1,000 square feet every week all season long. He had the greatest lawn ever.