IMHO, you can skip the overpriced specialty "washes"... I've never gotten poison-ivy or poison-oak since I started following the below "exposure protocol", unless I fail to follow the protocol (didn't know I was exposed etc)
The allergen that causes a reaction in ivy/oak is an oil, we need to get that oil off within a few hours or face the consequences. What do you ALREADY have in your house that is marketed specifically for removing tough greasy/oily residues? Hint: your wife likely uses it much more than you and it's not in the laundry-room. Yup, the dish-soap, name-brand, heavy-duty (not watered down) dish-soap. This WILL cut the oils if used properly. I do 2 wash-off routines as part of my protocol, the reason for it has to do a bit with science so I'll explain. Water is a polar molecule, it can act as a solvent, but only to certain things. Soap's best trait is the ability to bind up things that wouldn't normally bond to water, and allow them to be washed away. The problem is, sometimes water, introduced before the soap can be a hindrance, or in the case of oils, it can actually cause them to spread. However, with certain compounds the polarity of the water molecule can aid in the process. So my protocol involves soaping up on ALL exposed areas before getting wet and then again after getting wet. IE, if I've been cutting firewood and saw-dust laced with poison-ivy may have found it's way inside my clothing, my whole body could be exposed so I soap up head-to-toe before getting wet. Once soaped up and scrubbed really well with dish-soap I get in the shower and rinse it off very well. Then, I repeat the soap-up and scrub process in the shower. After that I shower as normal. I can buy gallons of Palmolive Professional Concentrate Dish-Soap for $5; I can go through a LOT of dish-soap for the cost of 1 single tube of Technu.