What do you want to get out of an internship? Do you want to get experience in other areas of theatre, or concentrate solely on stage management? Are you interested solely in opera?
I think looking through the internships already reviewed can give you a sense of what kind of things to consider. Even if a company you're looking at is not reviewed, asking yourself "Will this internship help me network? Am I going to learn something new? Do I want to work on a wide variety of productions, or concentrate on one particular kind of production?" etc will help you figure out what road to go down. Then, do research on the companies you're applying to. See if what they offer fits the experience you're searching for.
In terms of actually finding internships- there are many options out there. Off the top of my head, Glimmerglass and Wolf Trap have summer opera internships. A quick google search will find a lot more.