ESSENTIALS
Tuner app: Cleartune (iOS and Android)
Metronome app: Pro Metronome (iOS and Android)
Fantastic practice app + quickest self-recording device: Modacity (also has metronome + tuner)
Music theory, rhythm, and ear training exercises and tutorials: Téoria
SHEET MUSIC
Online library of public-domain music scores: IMSLP
Purchase digital scores: Sheet Music Plus
Go paperless with this beautiful app for downloading, notating, and organizing scores: forScore
rentals + music stores
Local violin and viola rentals + string instrument shop (604 S. Central Avenue, Glendale, CA): Metzler Violin Shop
Excellent violin and viola rentals within the US: Triangle Strings (Masters Series is best)
Shoulder rests, accessories, sheet music, strings: Johnson String and/or SHAR Music
technique BOOKS
Ivan Galamian - Principles of Violin Playing and Teaching
Simon Fischer - Basics