You get it free from grass clippings and leaves.
Other sources are: alfalfa meals, blood meal, fish emulsion, fish meal, guano and soybean meal
Nitrogen also stimulates vegetative growth as in healthy leaves etc.
For that I usually buy something like Bonemeal.
Other sources are: rock phosphate, colloidal phosphate.
Phosphorus is necessary for fruit and flower formation and rapid stem growth in seedlings.
Potassium helps to form stronger roots, stems and deepens the flower colour.
You get it free from wood ashes.
Other sources are: granite meal, greensand and sul-po-mag,