Companies

DJI: http://www.dji.com
DJI SDK: https://developer.dji.com
Maverick Drone Systems: https://www.maverickdrone.com

Training Sources

AOPA: http://www.aopa.org
DARTdrones: https://www.dartdrones.com   (on Shark Tank)
Fly Robotics: http://www.fly-robotics.com/
Mr. Steele: https://www.youtube.com/user/MrSteeledavis (FPV expert)
Small UAS: http://suas.modelaircraft.org (many other good links from here)
TV Specialists: https://www.youtube.com/user/TVSpecialists  (DJI Training)
YouTube (Dennis Baldwin): https://www.youtube.com/user/dennisbaldwin