I need a rear drive shaft right hand for my Forester.
Does anyone know:

1. Where can I buy one?

2. Can I get one for less than £400?

what model forester, what year, engine,transmision etc .
You certainly can get them for much less than £400, used original can be good way go, be careful with aftermarket and refurbs as many are garbage even compared to decent used .

I only use aftermarket when totally stuck on options as get too many issues such as noisy or vibrations straight out the box or within few months plus joint boots can be awful quality/fit and end up split or popping off .
 

Rear going be bit more forgiving on what used than a front would be, what the reason for needing a complete replacement shaft ?