How do I find an internship that involves physics?

To be honest, I'm not certain there are a lot of physics internship opportunities for high school students. But there might be special "summer programs" for pre-college students. I'd check the web sites for Lawrence Berkeley Laboratory and Lawrence Livermore National Laboratory (both in the East Bay) and for Stanford Linear Accelerator Center and NASA Ames Research Center (peninsula and South Bay, respectively) to see what, if anything, they list. (I know most of them have internships open to college students, but my recollection is that even those are more geared to college juniors and seniors and perhaps first-year grad students. At that level, you can consider options nationwide--there are many NASA centers and national laboratories; the latter include Fermilab, Sandia, Oak Ridge, Argonne, Los Alamos, and Brookhaven, as well as LBL and LLNL. I'm sure I've forgotten several.)

You could also check out local planetariums and science museums and see if there are any volunteer opportunities; it's not the same thing, but it is a way to start learning some aspects of the field and to start building up a network of contacts who might be able to help you in a stepping-stone sense. Both Foothill College and De Anza College in the South Bay have planetariums, as does the Academy of Sciences in Golden Gate Park. I'm 99% certain there's one in Oakland, too, but its name escapes me.

And take a look at what pops up for a Google "physics internships" search; while you might not be able to take immediate advantage of those listed by the APS or SPS, for example (https://www.aps.org/careers/employment/internships.cfm and https://www.spsnational.org/programs/internships), perusing the list will give you an idea of what to expect when you are eligible.

Finally, don't be too discouraged; I had a similar experience as a high school student (I wanted a job in a computer-related field but had to settle for a public library), but eventually I did internships at Fermilab, Argonne, and Bell Labs in New Jersey. All of them were outstanding experiences even if none of them were exactly the direction I expected to go. (Oh, and a physics professor hired me for a part-time position in my sophomore year; I did grunt work related to magnetospheric data analysis at first, as well as some programming, before moving to his rocket lab and helping construct sounding-rocket payloads by my senior year.)