Great question! I also haven’t been able to find any use “in the wild” from a quick Google search. There are examples in literature, but it doesn't seem to show up in books that were published before The Wire premiered (according to Wikipedia, in 2002).
That doesn’t mean it doesn’t exist—subcultures can easily develop weird word usages (potentially as a way to mark insider status). However, The Wire is fiction so I would be inclined to discount it as potential evidence for the existence of this construction in real life. (Here is a GameFAQs discussion that seems inconclusive to me.)
“X is/'s good police” sounds odd to me as well, and my current opinion is that I wouldn’t use it unless I was specifically trying to reference The Wire (which I haven’t seen, so I would pretty much never use it). I wouldn't say I find it "ungrammatical" so much as "unnatural": to me, it feels like a contrived way of speaking meant to produce some kind of special/rhetorical effect, similar to stuff like "After I collided with an animal, I had a lot of work cleaning deer off my car" or "because politics".
“Good police” is definitely used deliberately, multiple times, in The Wire, but I haven’t found any explanation from the show’s creators talking about the reason for this word choice (and if one existed, it would probably be of dubious value as evidence anyway: compare the dodgy origins of the phrase “clockwork orange”). (Here's a book I found with a chapter discussing the meaning of this term in the context of this show: "The best of Boys and Lads," by Kenn Fisher, in The Wire and Philosophy: This America, Man, edited by David Bzdak, Joanna Crosby, Seth Vannatta.)
It also shows up in a number of articles I found that specifically reference The Wire.
According to a Slate article, The Wire supposedly tried to represent some details of Baltimore speech “faithfully”, but it’s obviously not a documentary, and it’s unclear to me if this particular phraseology is the result of research or invention:
There is also the challenge of following the localized black dialect that the program tries to represent as faithfully as it does its other details. In the Baltimore ghetto, yo is both a salutation and the third-person singular pronoun; "feel me," means "listen to what I'm telling you"; and the ubiquitous use of bitch has mostly replaced the N-word. The cops have their own language as well, in which a capable officer is "good police," bystanders caught in the crossfire are "taxpayers," and young boys up to no good are called "hoppers." The dialogue becomes easy enough to follow after a couple of episodes, but first-time viewers should switch on the closed-captioning feature for the first hour or two so as not to miss anything.
(“The Wire on Fire”, by Jacob Weisberg)
I only found a few uses of “is good police” that didn’t explicitly mention The Wire.
One is an unsourced claim in a Mother Jones article:
Cops like to talk about “good police.” They say, “That guy is good police”—a top compliment, by which they mean cool under the pressure of the street and cunning at getting people to give up the details of a crime.
(“Police Shootings Won’t Stop Unless We Also Stop Shaking Down Black People”, Jack Hitt, September/October 2015)
However, no source is provided for “Cops like to talk...”, and since this article was written after The Wire aired, it might very well just be based on the portrayal in this show.
The other example I found is in the description of a novel on Amazon that seems to have been published in 2009 (so again, post-The Wire):
Gus Ramone is "good police," a former Internal Affairs investigator now working homicide for the city's Violent Crime branch
(The Night Gardener, by George P. Pelecanos)
The contracted form "He's good police" shows up on Google Books in a few novels published after 2012. For example, Google Books also shows the following interesting, but not really conclusive dialogue in the novel Another Thing to Fall (Laura Lippman, 2008):
Tess shook her head. "I won't second-guess Tull, or get in his way. He's good police." "People really say that?" "Say what?" "'Good police.' I've heard it on television, but I thought it was pure affectation." "It's what cops in Baltimore call themselves. Police, a police, a murder police. Where do you think the television shows got it?
(It's unclear if "It's what cops in Baltimore call themselves" has any more basis in reality than The Wire, since this is a novel.)