Abebooks is always worth a shufti (first port of call for anything obscure) :
AbeBooks: New Books, Secondhand Books, Rare Books, Out-of-Print Books .uk
AbeBooks: New & Used Books, Textbooks, Rare & Out of Print Books .com
Provides links into many specialist retailers too.

Cheers,
Adam.

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"Wars cannot be fought with dream stuff" - Sir Percy Hobart.