Repair:
Seattle Sound Repair
Pope Repair - Boston, MA
Patterson Hornworks - Las Cruces, NM
How to ship a horn

Supplies:
Kennelly Keys - Lynnwood, Everett, Bellevue
Ted Brown Music - Seattle, Tacoma, Yakima, Silverdale, Puyallup, Richland
Pope Repair
Al Cass Valve oil

Horns, New and Used:
Quinn the Eskimo  - Redmond, WA
Woodwind and Brasswind
Pope Repair - student-pro level horns, new and used
International Horn Society Marketplace - used horns

Books/Music:
Houghton Horns - Recipes for Success
IMSLP - public domain music

Practice:
How to practice - suitable for beginner and intermediate players
Molly Gebrian, Music and the Brain - thorough source of info about different types of practice including videos. Highly researched and compelling

Practice apps:
Tonal Energy - (transposing) tuner, metronome, recording device, drone, sound analysis, etc.
Modacity - practice app with lots of practical capability
Beat Station - scales are way more fun with a groove!

Performance:
Bulletproof Musician - so many great blog posts and courses about optimal performance and practice from Dr. Noa Kageyama
Sound in Motion, David McGill - taking the mystery out of "musicality"

Mindset:
The Inner Game of Tennis, The Inner Game of Music - the classics
Peak, Anders Ericsson - how to get good at things with deliberate practice