Draw a line down the middle of Metropolis and then cut it again into quarters like a football pitch. You want to look mainly in the 3rd quarter which is to the right side of the map, not quite as far as CEO Dam. Go up and down that 3rd quarter section and you should find most of them there. You can also check the broker as people sell collections there as well and that's a good way to fill in the gaps for the ones you have missing.
