Get familiar with the NALP website. Search firms by city and check out how many SAs each firm takes and where they recruit. This will give you a rough picture - I've definitely seen at least some that recruit from USC and not UCLA, and vice versa. Then search for each school's NALP employment report, where you'll find some info regarding SA positions (and other employment info) with some location data too.

In my research as a 0L, this method has produced the clearest picture, though it's still just a rough one. If anyone has any insight into finding more data, I'd also love to know.

And yes, as far as I've found, most large firms in LA will pay SA at the top market rate.