Summer and academic year research positions are often available in the Linfield physics department. See any physics faculty member for information about applications and deadlines.
To conduct summer research at another institution, consider REU programs at various universities, summer research at a Department of Energy national lab, or one of the many other possibilities. The Society of Physics Students maintains an extensive list of summer opportunities.
The American Institute of Physics also lists employment opportunities-- these tend to be for more permanent positions rather than internships.