If you got less-than-average results in you Bachelor/Master program, you should seriously consider not to do a PhD. While you might find an advisor if you try and look long enough (at a small or unimportant university), it is not unlikely that you eventually fail to finish your PhD and just wasted some years.
If you have problems in understanding difficult concepts in physics (which is often the reason for bad marks), then it may be not the best idea to enter an environment where people expect you to learn a lot of material from different sources without much help from anybody.
I saw somebody dropping out of a PhD program after 7 years, which is not really desirable.