Take a look at the Bachmann spares web site. Full range of bits, fairly priced available by return of post.
(Sorry, can't do linky thing off my phone, will edit later when I get home)
Usual disclaimers.
On buffer spacing, it should not be too hard to do 2 different versions of a 3dp loco body, buffers at oo spacing and buffers where they should be. These things tend to be print to order so extra costs are minimal.