I note what you picked (newspapers and news magazines, plus talk radio and Fox). But you left out MSNBC, CNN, and even the New York Times for some reason...
That is: Yes, there are center-right or right-wing press organizations. Despite that, it is still true that the bulk of the "mainstream" press is no longer center - they have become biased, and sometimes even actively helping Democratic campaigns. That's not what "mainstream" used to be.
The United States traditionally has enjoyed a wide variety of opinions in its various regional daily newspapers, and some studies in the past have shown very little bias overall (http://onlinelibrary.wiley.com/doi/10.1111/j.1460-2466.2000....). Obviously there was individual bias, but not in the meta.
Maybe something has happened of late (would love to see data, aside from cable news which I think has obtained strong bias of late). But personally, I believe that if there is a problem, it is more the consolidation of media outlets into very large corporations.
This is a lack of diversity problem, not a bias problem. If "mainstream media" is defined by Big Corporations, you have to include Fox News (News Corporation) and AM talk radio (largely Clear Channel) with the liberal side of MSNBC (Comcast), Time Warner (CNN), Disney (ABC), and CBS. I can easily buy that the fact we have much fewer voices than we used to driving the narrative might lead more to monolithic opinions and groupthink.
Incidentally, I personally find the charges you are making about media actively helping campaigns a bit weird as well, given Steve Bannon's role in the Trump campaign.
I never considered Breitbart as part of mainstream media, so I ignored that. CNN is in a completely different position than Breitbart.
"Mainstream" used to be NBC, CBS, and ABC, plus AP and UPI. Of those, UPI's gone; NBC, CBS, and ABC have more-or-less abandoned their role. Fox and CNN are new-ish (at least to a person of my age). So to claim that "the mainstream media" all abandoned the center means "almost all the original ones did" - not counting Fox (or even CNN) in the reckoning. Never the less, CNN and Fox are far more mainstream than Breitbart, which is a marginal fringe player.
That is: Yes, there are center-right or right-wing press organizations. Despite that, it is still true that the bulk of the "mainstream" press is no longer center - they have become biased, and sometimes even actively helping Democratic campaigns. That's not what "mainstream" used to be.