Sadly there isn't very many
If I was to rate like all the porn sites on how anal only they were, I would say LegalPorno is probably the most anal-only-friendly website. Most of their content is anal heavy and lots of the scenes by Giorgio specifically tend to be anal only. I think he makes a DP or mostly pussy scene like every other scene, but I would mostly say he's on the anal side of things.
Holed or Analized aren't bad choice either if you want more one-on-one or higher quality videography. Neither is truly anal only, but anal specific. Vogov is a new studio by Markus Dupree, which is predominately anal. I've only seen a few scenes, and they're pretty good, but I'm not sure how anal only it is.
Tushy is another anal specific website, although only a handful of their scenes are actually anal only. I like their quality and content, but I just wish it was more anal heavy, because often you get way too many pussy heavy scenes.
Teensloveanal, Ass Traffic, True Anal, are some other choices, but kind of a mixed bag in terms of quality and good anal content. True Anal is probably the most professional, Mike Adriano being the director, but he really just makes the same scene over and over again, plus he's been doing way too pussy fucking for my likes
Other than that you have some of the more mainstream studios, EvilAngel, JulesJordan, Brazzers, who make anal only scenes like once in a blue moon. I would say Brazzers probably makes a descent amount of them, just look for any one-on-one, anal scenes without a plot.
Particularly good is this scene, in which is a POV scene where the girl is locked in a chastity belt, gives "you" a blowjob then turns around to get fucked in the ass. Sadly, it's the only one I have ever found in such style.
Legal Porno has the tag 0% Pussy, in which the scenes have - you guessed it - 0% pussy penetration. There is quite a lot of stuff there, but since many of them are gangbangs with double or triple anal penetration, it's not exactly my cup of tea.
Also worth mentioning is Teens Love Anal, having quite a few scenes in which the girl wants to try anal sex in order to keep her virginity.