i need scrap iron for the marleybone crafting quest but i can't find any anywhere does anyone know where i can find some
Many, many, many posts here have dealth with finding scrap iron. I'd search these message boards for scrap iron.
On the other hand, you should find if you take the time to look in nooks'an'crannies as you progress through MB, you should have enough. (This assumes you are serious about this and are not looking for it at busy times).