Try this
Amazon.com: Tecnu Extreme Medicated Poison Ivy Scrub (4 oz): Health & Personal Care: Reviews, Prices & more
it works very well to cool the itch. Also, if you know you got exposed right away, it does help clean it up (it has an abrasive in it, scrubbing the skin) if you apply right away.
I would be careful with high percentage (prescription) cortisone over large areas of skin. That stuff soaks in and can mess with your body - it is a pretty potent steroid.
So, my vote is Tecnu and suck it up. If cortisone, go to the 1% OTC.
G