I got my first iron by digging patterns of trenches at the edge of the swamp, but be warned that this is a VERY labor-intensive way of getting what usually turns out to be only three or four pieces of iron per find -- and the finds are rare. Iron is the main resource choke point in the game. You'll have to find a large swamp with a number of sunken crypts before you can get a workable quantity of iron. On the other hand, once you do find the crypts, they can have a lot of iron in them -- I think my record was 121 pieces, and I doubt if that was the maximum.