I'm so glad somebody finally brought this up!
So right now LegalPorno is the only mainstream porn site that does openly advertise that a portion of their videos are anal only! On top of that anal really is their specialty!
Mike Adriano and
Jay Sin over at Evil Angel are anal orientated directors. They've come out with some interesting stuff, but their material is more on the fetish side of anal. Jay Sin primarily does big toys, insertions, and trans stuff. It's pretty cool, but sometimes you just want some regular ol'anal fucking. Mike mostly does gonzo, POV stuff. Mike also has some anal only choices, just look at the box art. Although to be honest he kind of lost of his touch and has been doing a bit too much pussy stuff and sneaking pussy fucking into his anal only stuff, along with some really weird shit like wearing mittens during fucking.
Examples of Anal Only, Mike Adriano box art.
Brain Pumper who previously worked for Evil Angel did a lot of anal stuff and even created an all anal series called "Gapeman." It's a pretty okay series if you're into gapes, but the production gets cheaper and cheaper after each installment. What makes these special is that fact that Pumper's character, GapeMan, basically states from the very beginning that he "don't want NO PUSSY."

Pretty hot, although the character is stereotypical and annoying. A lot of the women also aren't too into it making the chemistry hit or miss.
Raul Cristian (aka Cruel Media) who I believe is currently working for Jules Jordan tends to do some very great anal stuff. His series "In Anal Sluts We Trust" tends to be all anal with pussy fucking usually only happening during a DP. The rest of Jules Jordan's Ass Factory material (the so called "
All Anal Company"

) tend to be trash with more pussy fucking than anal.
Jean Yves le Castel is a retired actor and director that basically popularized hardcore anal. Although not strictly anal only and kind of dated, there are still some gems to be found in his work. He worked and helped train some other great anal directors/stars like Manuel Ferrera, Nacho Vidal, and Rocco Stifredi. Although all three have lost touch with their anal roots

. Next is
Skeeter Kerkove who was all about the ass along with his ex-wife, Bridgette Kerkove. His material is more of a bargain to be honest with cheap women and production making up his repertoire. If you're into coke whores and plastic surgery botch jobs, but a great anal focus then I would go with him.
Some others off the top of my head include Straight to the A and Army of Ass.
I'm personally hoping to start my own Anal Only porn studio, although I'm waiting to get my degree before I get too serious with anything.