Have a look at this guide whenever you want to know where to farm for anything: Barry Farmz Here -- Comprehensive data driven farming guide
In case you’re having any trouble working with the spreadsheet that @zephyr1 linked to, which is an excellent resource for answering these types of questions, anywhere in season 1 province 6 will give you the best drop rate of crude iron for the WE. @BarryWuzHere recommends S1:6-8 because it also gives you the best drop rate for recruits, which will add up nicely if you are going to farm there a lot.
You also just missed a great opportunity to stock up during Atlantis Rises. S2:12-10N is the recommended spot for that.
Also, I edited the title of your post to state that you are looking for “crude iron,” which is the English translation used in the game.