I have 03 Accord too, and I already replaced all speakers with premium ones plus an amp; still keep stock head unit. For your rear speakers, they are 6x9 coaxial speaks, and you can find them almost every stereo deaker.

It is easy to replace them yourself. Lift up the cover speakers. They are a bit tight because they are secured by plastic snap-in nuts; 4 of them. Use a screw driver to loose them out.

You can also go to the trunk, look under rear deck, and push those plastic snap-ins up to release the cover. You also need to cut out the foam seal under the cover. The remaining part is so simple, and I guess you know what you are going to do.

Good luck!!!