Where do people use 来着 a lot? Is that typical of 北京 or where？
来着 seems to be a kind of substitute for 吗 (是几号来着？). Is that correct?
Well, I can tell it's been widely used in the northern areas. And you would hear it in TV shows. Of course, it's an oral term.
It's not used to substitute for 吗. It's been used when you can not recall something from your mind, and then you ask out while you are still trying to think it out. For example, 几号来着？ means you can't recall 几号, and then you ask while you are still trying to think it out. In this case, you are expecting someone could remind you while you're thinking. 几号来着？ is kind of like saying "那是几号了？"
It is used at the end of a sentence.
It represents the completion of a specific behavior or event.