International Music Score Library Project (IMSLP): A huge online collection of scores and parts of public domain music. You can search for works by composer, title, instrumentation, nationality, and time period.
The Orchestral Bassoonist:A website devoted to orchestral excerpts! The site includes sheet music for commonly-asked bassoon excerpts, recordings, musical analysis, and practice suggestions.
Trevco Music:"The world's largest purveyor of sheet music for double reeds!" This is my go-to site for most of bassoon sheet music purchases. You can search be instrumentation, composer, publisher, or special collections, such as "Female Composers" and "Composers of Color."
Oboe/Bassoon Duo Repertoire: An annotated spreadsheet listing over 50 works for oboe and bassoon duo! Compiled by Margaret P. Fay and Graham Mackenzie.
MUSIC THEORY
MusicTheory.net:Great site for drilling aural skills and music theory basics.
Music Theory Examples by Women:A database of musical examples by female composers. You can search by composer name, era, and a number of musical parameters, such as harmonic progressions, cadences, and form.
Open Music Theory:An open-source online textbook with lots of clear explanations and musical examples for university-level music theory students.