[Satan]

By no means should firearms be banned - just have a proper screening for anybody buying them, introduce a firearms licence or something. That's how it's where I am from, you have to get a licence to bear arms, including a really quick shooting test and a psychological screening (I think). And we still have 36.something firearms per 100 people (sure, under half of that in US, but still high on world average) Most people use their firearms for hunting and/or shooting range practising/hobbies. And that's (I think) why we have a lower amount of homicides with firearms.

Also, are there any statistics as to how many shooters (who commit homicide) get their arms illegally?

Depends on if you include gang violence.
Including gang violence, it is the majority just based on that they commit murder more often.
But your mass murderers usually obtain them through family members, or stealing family members' guns. I've personally only seen three cases of homicide where the shooter bought guns themselves. And just 2 of those were mass murders.