I don't really know nor I want to know about gun purchasing, but sadly you can get some of it online. There is this website which is like a firearms amazon, but sadly I forget the link of the website, since my friend just showed the thing and ya I checked the website as my friend showed it to me. But I'm pretty sure it requires some basic stuffs like identification card and more to purchase any guns from the website.

Internet is scary, yet it's hard to limit the use of it.

With that being said, I think that a solution with this kind of matter is very unlikely to be found right away, but maybe in the future as people already aware of this kind of threat there will be options to maybe reduce the amount of this case.