Some places are obvious - local hobby shops if your lucky enough to have one, Hobbycraft have a large choice of tools and accessories not only in the modelling aisles but also some useful stuff scattered around the other craft sections eh. wire, chain, tissue paper etc.
If there's neither then maybe look out for train or rc sellers which have a lot of overlap with plastic modelling.
DIY stores and even Wilkinsons etc can provide cheap sanding tools, tweezers, cutting mats etc etc.
Basically keep you eyes open as tools and accessories can show up in the most unlikely places.