Not sure if this is common knowledge but an absolute awesome person has provided the 3D print file Middle gear for the following devices: D-555, D-88, D-250/25, D-90/9, D-99, D-150/15 See link below: https://www.thingiverse.com/thing:5718683 Creator says that an SLA printer is recommended or even DLP resin printer to allow for the tiny teeth. A big thank you Stephen Lulz!
Or buy ready made from one of the vendors on e-bay. Saves having to buy the correct high quality 3D printer. Unless one already has one of course...