Oil seals are standard parts, and most of the ones Honda uses are available from bearing supply places like Motion Industries. Larger ones usually have the size marked on the closed edge, like 35x25x6 - this is for a 35mm hole with a 25mm shaft and it's 6mm thick, really small ones are hard to be sure which number means what as the order is not standardized. If yours doesn't have the marking you just measure the 3 dimensions. The really really small ones - for example in the speedo drive housing around the driving shaft is about 5mmx2mmx4mm - are pretty much unobtainable. Special seals with retaining tangs or rings (available only as OEM parts) are used in the engine cases where the crank pressure could blow them out - but only on the big seals with a large area for the pressure to work on: the small ones like the shifter shaft are plain press-in.
