Try Elmhurst Hospital in Queens, if you are in the NYC area. They have an excellent summer externship program, and they are still accepting applications in April.
If not, don't worry. Get ANY job in a hospital, or volunteer in the area where you would like to specialize. I am currently looking for jobs, and believe me, ANY hospital experience is helpful and shows that you are passionate about nursing. Looks way better than working at Starbucks.

Good luck!