Same problem here- I've been told by more than one seller that part 2 is out of print. I've seen it going on ebay for $30 and up. Hopefully one the Iron Kingdoms hardback finally comes out and stirs up some more interest in the world, they'll do another print run of the modules.
You may be able to get by with just 1 & 3; if you look at the reviews it seems like #2 is least essential, you could possibly fill it in on your own. Though things like the steamship cruise and Clockwork Temple would definately be cool.
There's also a pdf module that's supposed to go between parts 1 & 2, but I can't remember the title. It's over at rpgnow.com.