Most of these links will be via IMSLP, the International Music Score Library Project
https://imslp.org/wiki/Main_Page
This is an amazing resource for older music that is no longer under copyright. That means that all the great classical repertoire from the early twentieth century and before is available for free due to citizens everywhere willing to scan and upload it! You can browse by composer, time period, or instrumentation.
MUSIC THEORY
Toby Rush
Music Theory for Musicians and Normal People
https://tobyrush.com/theorypages/index.html
The Major Scale:
https://tobyrush.com/theorypages/pdf/en-us/the-major-scale.pdf
Key Signatures:
https://tobyrush.com/theorypages/pdf/en-us/key-signatures.pdf
The Circle of Fifths
https://tobyrush.com/theorypages/pdf/en-us/the-circle-of-fifths.pdf
Triads/Chords
https://tobyrush.com/theorypages/pdf/en-us/triads.pdf
Diatonic Triads (Roman Numerals)
https://tobyrush.com/theorypages/pdf/en-us/triads-within-tonality.pdf
PIANO REPERTOIRE
Bach
Selections from the Anna Magdalena Notebook
https://imslp.hk/files/imglnks/euimg/b/bc/IMSLP640945-PMLP06107-bach--notenbuch-anna-magdalena.pdf
Bartok
Mikrocosmos
http://imslp.eu/files/imglnks/euimg/2/21/IMSLP465640-PMLP3661-Bela_Bartok_-_Mikrokosmos.pdf
Burgmüller
Etudes
Clementi
Sonatina in C Major
Czerney
Practical Method...
Streabborg
Twelve Very Easy and Melodious Studies for Piano
Sheet Music Plus
Local Bay Area business to purchase music online
https://www.sheetmusicplus.com/
I’ll continue to update this list periodically. Stay Tuned. Thanks!