Considering that around 50 police officers are killed with firearms in the entire USA annually, I'm a little skeptical that any meaningful data went into the breathless charts in this article.
Here is a very nice, detailed, interactive map with information about policemen killed by guns over the past decade:
A quick perusal shows that 41 (out of 511) deaths were with a firearm purchased on the black market, 77 were stolen, 107 were legally purchased, 46 were borrowed (not sure exactly what the details of 'borrowed' were), 16 were a straw purchase, 51 were department issue (I'm assuming this was a case where the officer was killed with their own firearm).
California (very strict gun laws - 47 deaths) had almost as many police officers killed as Texas (very lax gun laws - 46 deaths). Yes, I know, California has a much larger population than Texas.