Vitamin D is produced from the irradiation of cholesterol by UV light.
Northern latitudes (i.e. New York, Chicago, Seattle) get less UV light, especially during the winter when the Northern hemisphere tilts away from the sun.
Real eggs and butter contain cholesterol and provide the building blocks for Vitamin D production in your body. Do your research and be skeptical of the mainstream consensus. There is not consensus in the medical community that cholesterol should be avoided.
Get sunlight daily. Treat it like your morning coffee. Take off your shirt and enjoy some time outside in the morning and late afternoon, avoiding the midday burn. 15-30 minutes a day is quite pleasant. Combine with a sport or some exercise.
Plants need sunlight to produce energy. Humans need sunlight to produce Vitamin D. The sun is not an evil, cancer-producing orb in the sky. Like anything else, it's all in the dose.