drug to cause paralysis
I'm writing something set in modern-day America and I'm looking for a drug/venom that would cause paralysis when administered. I'll take whatever I can get but bonus points for one which would cause paralysis from the neck down only (if such a thing even exists). I'd prefer it if the drug wasn't too difficult to obtain so several people could be suspected of administering it/being able to have it in their possession.
If such a drug does exist, how much would need to be administered to paralyse an average person, not taking into account obvious age/gender/weight differences?
I've tried Googling various terms relating to paralysis, drugs, snake venom and toxins, but in most results the paralysis is either vocal cord paralysis (which doesn't interest me) or relates to snake poison in Australia.
Thanks for any and all help.