There are a few fairly expensive, individual figures around, but I tend to use converted figures, in order to achieve what's required for a particular diorama.
The various sets from Tamiya are very useful for this, and ignore the 'nationality', as plastic figures don't talk (unless I've had too many in the pub!).
They can be converted with just paint, or the addition of Milliput, either in their moulded 'pose', or their 'pose' altered, which is quite easy, as most have separate limbs etc.
The same can be done with the aviation orientated sets from Revell, ICM, Dragon and Eduard etc.
Note though, that the Tamiya figures in particular, although very nicely produced, tend to be of relatively small stature, compared to, say, a figure from a Monogram kit, and I'm guessing that this is a result of being modelled from what is available as a pattern - a live person, with the average Japenese being smaller in stature than, say a European or American person.
The photos below show some Tamiya figures, from the Panzer Servicing crew, I modified for use with my RAF Boston and 361st FG Bottisham dioramas.