You should first of all find out if your own health insurance can offer you a package covering your stay in the USA. You must remain insured in Germany so that you are covered whenever you return home.
Nearly all insurance companies offer foreign health insurance, rates for which you can find on their websites.
Since the USA belongs to the NAFTA bloc, premiums are somewhat higher than in other countries.
Here are some examples of insurance companies offering long-term health insurance (longer than 56 days) for the USA:
The company Mawista specialises in foreign health insurance for those studying abroad.