Yes, the KOP sensors are custom for FIRST so you can not purchase or use more than the original quantities, for the team.
A reasonable thing to do would be to ask other teams in your area if they're not using the KOP sensors, if you could buy or swap parts for them. We've swapped parts with teams in the past to get more KOP sensors for testing and practice robots. There is also the "I Have / I Need" feature here on CD.
Lastly, you can purchase sensors at reasonable prices from several vendors. We have used
Trossen Robotics,
Robot Marketplace, and
SparkFun Electronics to name a few.
SparkFun has numerous accelerometers and gyros which will work directly with the FRC control system.