Perhaps you have found labs staffed with something other than chemists in one flavor or another, but mostly I believe that is who (or suitably trained lab assistants of course) is responsible for laboratory water testing. The lab would also be the easiest to find place that probably has a series of calibrated filter media to determine particle size.
The new poster has said nothing about tests except that he has iron. He further suggests from the color of the water that additional materials are present. If there is a particulate problem a lab could determine what pore size would be required to eliminate it. In the long run this could be cheaper than buying a filter and trying different media until a suitable one is found; or not found.