In this page, you can find websites that may help you to learn and advance your performance skills on your choice of instruments!
(You may also found that there are more percussion-related materials because as you know, I'm a percussionist(●´ω`●) )
You can also find pieces (sheet music) that is suitable for your performance level!
If you don't have a copy of your own yet, this is the book that you should have as a pianist for finger control. (Free download from IMSLP)
Good method book for beginner Jazz piano playing. Take a look if you're interested on playing Jazz piano! (Free download from siteape.org)
For beginner to advance guitarists: Good website for free guitar lessons and pieces!
A must-have method book for trumpetists at any levels! (Free download from free-scores.com)
Good method book for students who want to start learning the drum set!
Practice rudiments for percussionists (drummers and keyboard percussionists).
A good method book for practicing snare drum! (Free download from DLSCRIB)
A must-have method book for beginner to intermediate level percussionists (drummers)! (Free download from DLSCRIB)
A must-have method book for beginner to intermediate level percussionists (keyboard percussionists)!
Good etude collection for beginner ~ intermediate-advance drummers to have! (And I struggled on it when I was a student) (Free download from DLSCRIB)
A good website to search for Jpop/Anime/Drama and some Kpop official sheet music to perform. Since they're official published they'll cost money to obtain the files and/or print them out in convenient stores, but it also means you could use them as secondary or even as primary source for your research!