your whole sound system is from poineer. that is not replaced, it is what comes with factory. Your head unit, amp speakers... they are all pioneer designed.
as for replacing speakers, any speaker 5 1/4 actually should work. I doubt its ganna match factory sound becuase the system was fine tuned to work very well with each other. nonetheless, you shouldnt have a problem replacing them. the rear size is 6x9. trying to obtain a factory speaker, you can maybe try a junk yard or something. Its worthless to buy it from a dealer because they will just rip u off for that. aftermarket will be a better speaker anyways
-neil