Here you go - most extensive official Leica service manual for the IIIf (the other Barnacks are all less complicated, so with this manual you can do simpler versions as well) - it has also a complete parts list:
http://www.pentax-manuals.com/manuals/service/leica_iiif.pdf
If you want to repair Zorki's & Feds or other Sovjet cams, you better use the Isaac Maizenberg manual:
https://www.mikeeckman.com/media/MaizenbergSovietCameraBible.pdf
Precaution:
Please note that especially the screws on Barnack Leica's are very tight: so in order to prevent stripping the screwheads, use the right screwdrivers and if necessary, sharpen them! (the screws are made from relatively soft brass).