Help getting a dead reckoning

I've been playing for a few weeks now, and I've got a witch at level 49. The skills I've been using are starting to get out leveled and i heard that dead reckoning is a really good gem for late game. Where can i get it?

Like most uniques in this game Dead Reckoning is a random drop and your best bet is to save up some currency and trade it from another player.

http://poe.trade/search/iobatoriaminar <- if you're playing in the harbinger league people seem to be asking 5 chaos orbs for them.