Just looked it up at Fishers (Suppliers)
and it's surprisinly cheap. Around 20 quid for 500g.

*Throws bottle to Benny*

Edit: Hold on, the good stuff is more:
Potassium cyanide 99+% for analysis 25g CertiFied AR £347.36
( Hankster, Thu 9 Sep 2004, 15:34, archived)