This is a nursing forum so there are only occasional visits from actual Physician Assistants (PA) around these threads. Your experience as an RPN in Canada will help in meeting the requirement for "Health-care related experience" prior to entry in PA programs. There are a variety of educational entry points to becoming a PA. While the curriculum is essentially similar, PA programs are either Associate's, Bachelor's, or Master's degrees.
There are very few Associate's degree programs in existence. Since you mentioned interest in Northern California programs, there are 3 around here: Stanford, Samuel Merritt, and University of California - Davis, and all 3 have Master's degree programs for PA studies. That means you have to enter with a Bachelor's degree in something already. You also must have completed prerequisite science courses as well as probably have to take the GRE. http://www.paeaonline.org/index.php