I need to store a 2d matrix containing zip codes and the distance in km between each one of them. My client has an application that calculates the distances which are then s
Stephen C. has a good point: if the distances are as-the-crow-flies, then you could probably save memory by doing some calculations on the fly. All you'd need is space for the longitude and latitude for 952 zip codes and then you could use the vicenty formula to do your calculation when you need to. This would make your memory usage O(n) in zipcodes.
Of course, that solution makes some assumptions that may turn out to be false in your particular case, i.e. that you have longitude and latitude data for your zipcodes and that you're concerned with as-the-crow-flies distances and not something more complicated like driving directions.
If those assumptions are true though, trading a few computes for a whole bunch of memory might help you scale in the future if you ever need to handle a bigger dataset.