Sorry to hear about the bad news
I'm not sure what all the American terminology means -- what's a "rising physics senior"? Senior = 4th year right? Does that mean you are going to go into your senior year next fall, or does that mean you're about to finish your senior year? Or something else?
Some options to consider:
1. (This advice is specific to how my undergrad University works, I'm not sure if it's the same elsewhere). You could delay graduation. Stay in undergrad for another year and maybe take a few grad courses or senior physics courses that you think might help you. More importantly, take the "independent research" or "honours/senior thesis" courses so that you get more research work done. I would take the minimum amount of courses to keep full-time status. At my school, you pay tuition on a per-credit basis so if money is an issue, then just take one course (the research course) and switch to part-time status. With only a few courses, you might also be able to work a part-time job on the side to make money or volunteer in the department.
2. Alternatively, you could still graduate then re-enroll in your school as a special/non-degree student. This will allow you to take the research courses and work on research. Of course, you would have to check how this works at your school and make sure you are able to do it!
3. Volunteering at a P&A department could work, however, it's much much easier to be volunteering at your own school, where people know you. Are you able to stay at your school this summer and work/volunteer with your own department? Many schools that I know of (Canadian ones though, so may not apply), like to hire their own undergrads for the summer after they graduate.
The best advice I can give for getting volunteer work at a department that doesn't know you is to just "cold-call" them. Just go and do it! Many will probably say no -- free work is complicated, but you just need one "yes". I would email them soon saying that I will be in the area this summer and that you would like to gain additional research experience by working in their lab. I would write a 1-2 paragraph summary of my experience and maybe attach a CV (or ask if they would like to see it).