You can find one on GMTower and YukiTheater (Cinema). I've been to many servers before and found out that you can find pianos on Roleplaying (Half-Life 2 RP and Dark RP) and a few Cinema servers.

Half-Life 2 RPs is what I'd recommend going to for playing a piano in front of people nowadays.

Dark RPs are alright, but not preferred unless you can tolerate trolling/griefing. Sometimes you'll be required to become a donator, or buy any "Pianist" class/job just to have access to the piano. It's not worth it.

Cinema servers used to have pianos everytime you visit one. Sadly, I can't think of any other place other than YukiTheater that has a piano anymore.
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That's pretty much it, happy piano-playing~

IP: 74.91.119.29 DarkRP server with a pianist job

Thanks!

DarkRP Server with Pianist Job (open for all):
51.254.141.108:27015

GambitRP

Le Lycée des Thugs