The big question :-) I know, it's a tuff one!!
When i decided to learn the piano last year I came up with the same question. Surfed the net to find answers but...difficult. In Belgium, you have 2 options as an adult: you find a private teach or you can go to (adult)school at the evenening. First one charges you per hour and you get private lessons. 2nd option, the school, is 2.30 minutes music theory and one hour piano lessons in groups of 2-3 players.
I was advised to get a private teacher, as "you would play/learn piano for one full hour instead of 15 minutes when in group... if you would do that for one year, you would have learnt "more" than at school."
After a year, still a struggele for sight reading, I deciced to go out and look for a 2nd teacher...so I would be able to swap (one week teacher A, other week teacher B)... well, for months now i'm still searching for that second teacher! Its very hard to find private teachers willing to give lessons at the evening or weekend. At the end, i now subscribed at this music school ..so on wednesday theory, thursday piano lesson in group, and saturday piano lesson with the private teacher.
I've contacted my piano shop, artists etc etc to find a 2nd teacher, but... still no luck. I think i was lucky finding my first one..so i'll surely keep him

I've the impression that there is only some kind of education for "real" musicians... kids playing as of the age of 4...growing, going to conservatory, entering master classes etc...
I know, I will never be the pianist I would like to be, but i'm interested in music study, piano etc, so I don't see an issue, I even pay for this education...but i'm not given the possibility to do it.
I'm very interested in the piano... I even thought about going to conservatory (real music study) but you have to do examinations to get in... how can someone even do an examination if you can't even find private teachers first, decent music/piano teaching??? That's so frustrating... so I try to read/learn about music myself but that's not an ideal situation of course. I can become a lawyer, MD or whatever whenever I want... but not with music

I wonder, in ohter countries, do adults have the possibility to go to a kind of music university or so and start a real music study? If you're young, as of 18 you can go to the conservatory or university... if you're "old"/adult and want to do this... university no issue, but music study... forget it... that's a pitty. Would really want to do this...
And now comes the question...what is a "good" piano teacher if you just start playing the piano?? I have no clue...