Poison ivy / poison oak / sumac has a pretty intense chemical that requires a very little amount to cause problems — the average [accidental] dose is 100 nanograms. It contaminates surfaces invisibly, and will remain potent for 5 - 50 years. And it grows EVERYWHERE.
So it could be used for evil purposes.
To harvest and purify urushiol:
- collect the sap  from a plant in the Toxicodendron family
- dissolve the sap in acetone
- pour off the acetone, and discard the insoluble part (referred to in various places as "acetone powder")
- evaporate the acetone at room temperature
(You do NOT want to heat this, because the vapors are nasty. If you have nice lab equipment, you can evaporate more quickly under a vacuum.)
- what's left behind is urushiols
(other possible writeups about extracting urushiol:
Urushiol rashes can get pretty severe
so protect yourself when handling the stuff, and be aware of the harm you can cause.
Note that it can take 24 - 48 hours from exposure until a rash develops, so this isn't a fast-acting weapon.
If you were REALLY evil, you could burn or otherwise vaporize urushiol, which will give someone a rash inside their lungs. However, this is known to be fatal. So yeah, don't do that.