A website I forgot to mention is Crutchfield. They have a speaker fit guide that goes all the way back to 64. For newer cars, they have a scale to tell how difficult the installation would be.

Another option for SLL's son would be to get some small home theater satellites and glue/velcro them in somehow. I would say find something very small for these, then get some 8 inch midrange/highrange bass speakers and a sub. These can be found in normal car audio equipment without too much trouble.