I have alot of experience with Sprags. I am the Chief Inspector a company that works on helicopters. Helicopter component overhaul is what I used to do for a living and they all have freewheeling units to allow them to auto-rotate if the engine lets go.
To run one, it needs to be between the transmission and the driveshaft or the driveshaft and the rearend. You could probably get a sprag out of a used helicopter if you called a operator, or a service center and asked nicely. They have relatively short retirement lives in the aircraft and are removed in great shape and tagged unserviceable.
The problem is the size needed. A turbine engine puts out a steady state rpm and torque. To govern the power of the aircraft you change the collective pitch of the blades. The RPM stays constant.
A Bell 206 Long Ranger will have around 400 shp and a Bell 407 will have 500+. Those will be the most common sprags to run across. If you ask you could probably get the inner and outer shafts as these have very minute damage limits. I could email the copies of the maintenance manual pages needed.
The best might be out of a Bell 412EP. 1200SHP out of a Pratt and Whitney PT6T-3DF Twin Pac. The clutch is in the engine combining gearbox and doesn't have the same retirement life as the other aircraft. It would be harder to come across. You would probably need to contact Pratt and Whitney Canada or Standard Aero and see if their overhaul shops have any laying around they could release.
For a smaller unit, contact your local PD, News, or other operator and tell them what you want to do and why. You will probably have to sign some release form as the FAA has gotten strict on used-unserviceable parts showing up later on installed on an aircraft.
C. J.