Hi,
I posted something on the gear case shims a couple of pages back:
The genuine Mugen gear case shims (according to Mugen manual 13.5x15.9x0.2mm) are available only as part of an expensive Mugen "washer set".
I found a few other shims at A Main Hobbies and Tower Hobbies, but they seem to be of different size:
Associated 16x13x0.2mm #89117
HPI 13x16x0.2mm (Hellfire) #86598
OFNA 13x16x0.2mm (Jammin X1) #40024
Team Magic 13.2x15.9x0.22mm #130101
Kyosho 13x16xthinmm/13x16xthickmm (MP777) #BS53
Thunder Tiger ?x?x0.2mm (EB4 S3 buggy) #PD1896
Hope that helps.
Best regards,
Frank