Hi John,
I think everyone would like to find an internship like that! I am going into my third internship this winter, so I can give you some of the advice I have received over the years. One place that I was referred to that has been really helpful has been aviso. The web address is: http://aviso.aam-us.org/. There are a number of job openings there. The other thing that I have done has simply been to find an institution that I am interested in working, and contact them a number of months in advance of my ideal start date. Most cities in the U.S. have a website that includes a list of museums with contact information. Rather than passively waiting for something to be posted, I have been able to, at the least, learn more about the institution, and have received internships that have helped both the institution and my education.
Hope this is of some service to you.
J.D. Frank