Most of these links will be via IMSLP, the International Music Score Library Project
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 for Musicians and Normal People
The Major Scale:
The Circle of Fifths
Diatonic Triads (Roman Numerals)
Selections from the Anna Magdalena Notebook
Sonatina in C Major
Twelve Very Easy and Melodious Studies for Piano
Sheet Music Plus
Local Bay Area business to purchase music online
I’ll continue to update this list periodically. Stay Tuned. Thanks!