Wrought is certainly more resistant to corrosion than steel, but it's not rust proof
Finding bigger pieces say for forging gun barrels can be difficult but not impossible. Smaller pieces are all over the place but you have to know what you are looking for and even when you find it some of it isn't suitable for forging
Sometimes I hear a rumor that there is a company overseas making wrought - I think what started this is there's a company in England that does sell wrought iron but they aren't actually making any, what they do is process old wrought and form it into different sizes of rod, bar, etc. Don't get me wrong it's probably good stuff, I just don't know of anybody making new wrought iron in quantity or for commercial sale