Scrap iron is only found in Marleybone or you can buy it at the Bazaar.
By the way? In the future just put what it is exactly you're looking for in your title. I was thinking you were asking about all the regeants. So I went through the whole list of posts looking for the complete lists of regeants and what spawned what, thinking you were curious about all the regeants. LOL!
I could have saved myself 15 minutes of searching if I'd just known you were asking about scrap iron itself, if that had been the subject line instead of 'reagents'.
I feel so daft right now for going to all that trouble over a single regeant.