Do not try to build all the structures at once, it’s better to follow the tasks of the chapters of the act and their requirements. During missions, you will be shown where to look for resources for the next building.
Until you start building the next structure via the mission, special materials (antennas, iron wire, etc.) will not appear on the locations. Either that, or access to the very places with resources will be closed. For example, the exit to the highway (at the gas station) will be blocked until the corresponding mission of the second act is launched.
Also, the drop rate of these items isn’t 100%. It’s not that everyone will definitely find them. If you didn't get the desired detail via the mission – don't give up, try once more!