Playbill.com is by far the best way to find them. That's how I got my two last year. You can sort by internships. Or know which companies you want to work for and try to get in touch with them. It's a little late to apply for them now for the summer- most of them have hired and are starting soon. Because of my internships last year, I was able to learn more about what I wanted to do and get connections for this year.
Hope this helps! If you have any other questions, let me know. I know there are a lot of other people on here who have done internships too.
"This table, he is over one hundred years old. If I could, I would take an old gramophone needle and run it along the surface of the wood. To hear the music of the voices. All that was said." - Doug Wright, I Am My Own Wife