Where could I take a High School Physics course online and how would I then get the 2 final semester grades onto my transcript?
For the transcript question, you would need to talk to the administration at your school. My D is taking (hopefully) the fast-paced high school physics class at CTY this summer and we needed the OK from the principal to enroll in AP Physics C for the fall.
We didn’t bother trying to get it on a transcript because we will have the CTY transcript. But they are an accredited school, provide the full syllabus, experience-based advice on how to work with school to get what’s needed, and will provide a grade if requested.
Their advice may be useful, wherever you decide to go - https://cty.jhu.edu/summer/about/credit.html - especially the linked Parent’s Guide pdf.
Some schools only allow you to skip courses from certain schools they approve, so you should probably talk to your counselor about this because they would know your school’s policies on summer courses the best.
About providers, most (I think all, but I’m not sure) of the kids in our school who took Physics over the summer either took it at a nearby school or at CTY.
It is up to your school/district if they will even accept anything taken elsewhere. If they do they can have specific requirements. Your best bet is to talk to your guidance counselor since honestly no one on cc can give you a definite answer. If your school is crazy now because of corona virus (many schools are closed/closing where I live) you might have to wait otherwise try to speak to or email your guidance counselor.