These do get listed on E-bay from time to time.
The 1997 catalog lists 62006 for older locos and contains 3 sets of gears and shafts.
62007 is for newer locos and is just 4 idler gears. Why 4? You need 4 for the large American diesels.
Nicholas Smith Trains in Broomall PA has them. I just bought a bag containing both axle gears and idler gears for my Mogul. They also have the motors.
NST has a great collection of NOS and when I was there two weeks ago had just received a large collection of LGB stuff.