Almonds seem to be an easier botanical source of cyanide. Wikipedia says ~50 bitter almonds can yield enough cyanide to kill an adult. I'd imagine you could make some kind of almond butter out of it and just eat it that way. Again, per Wikipedia, they contain an enzyme "which, in the presence of water [...] yield
glucose,
cyanide and the
essential oil of bitter almonds, which is nearly pure
benzaldehyde, the chemical causing the bitter flavor." So instant cyanide, just add water! And if you can remove the oil without also removing the cyanide (or just extract the cyanide) that'd take care of the bitterness. I have no idea where to buy bitter almonds, though. Also, if you can make HCN gas I think that's a preferable method over ingesting cyanide.