I can get
Cyanide.

*Waves keys to dangerous chemicals store*
( Hankster, Thu 9 Sep 2004, 15:27, archived)

GIMME!
*makes attempt to grab key*
( Benny Tied To A Tree As much use as Anne Frank's drumkit, Thu 9 Sep 2004, 15:28, archived)

*Yoink*
How much cash do you have?
( Hankster, Thu 9 Sep 2004, 15:29, archived)

*Looks through pockets*
84p and a button to a shirt I lost 6 years ago...

*looks up expectantly*
( Benny Tied To A Tree As much use as Anne Frank's drumkit, Thu 9 Sep 2004, 15:32, archived)

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)