Yeah, um. But:
It’s practically meaningless to ask where he got the gun, since anyone in Missouri can privately purchase a gun from anyone else with no questions asked. Missouri’s Republican leaders, like their congressional counterparts, have made sure of that by opposing universal background checks for all gun purchases.
Because the Post-Dispatch already has its point to make–the protests are peaceful, guns are bad–it doesn’t matter if the convicted felon and continuing criminal got his gun through the means–private sales–that the Post-Dispatch wants to flog or whether he got it through other means which are illegal and would not be stopped by more laws restricting the law abiding but not convicted felons and continuing criminals. The editorial writer does not need know whether he stole the gun, got the gun from someone who illegally purchased the gun for him, received the gun as a gift from a fellow criminal, or bought the gun from another convicted felon who should not have had the gun, either, and would not have run a background check if the law required it. Knowing it might have made the editorial impossible.
So, yeah, it’s not important to the canned editorial the newspaper opened.
In the real world, though, even if this law were in place, hobbling law-abiding citizens, this guy would probably still have gotten a gun.