Most of the time I do not rush Iron Working research. When I get it, my cities are reasonably expanded and several tiles around them are already improved. The problem is that when I have it researched, it reveals Iron sources only outside of my borders. However, when researching other technologies revealing other strategic resources (Coal, Oil, Aluminum etc) quite frequently I do get "X source revealed" and it happens to be inside of my territory. It isn't about size of my civilization, because I even do find Horses when the borders of my only city haven't really expanded. Haven't found a single Iron resource within my borders during all the games I have played. As a result I tend to rush Iron working in the hope, that it will spawn just outside of my territory and will get it by border expansion. Early game without Iron can be quite challenging if you happen to have fighting civilization as a neighbor.
Is Iron resource somehow special in this matter? How does the game spawn resources around the map?
I'm talking about vanilla Civilization 5 and typical setup (no changes in Advanced Options).