Is there any specific place the Exonerator drops or do i have to pray to RNGesus. The level 58 version
Just pray to RNGesus. There is only one version by the way.
Etram Fald in tyrants hold. Better chance since you can choose the type of legendary that you want. Thats your best bet.
Nice thanks 
Farming inc 
I wouldn’t consider it worth it tbh. There are much better options for a lightning pistol, or better yet, not using a lightning pistol in the first place. It’s badly supported weapon type, and often if you see builds going for lightning damage, you are either getting it through Shaman’s Primal Strike CD modifier (which must use a Rifle), Demo’s insanely powerful Static Strike, which has some of the best over-leveling in the game as far as value of points goes (meaning you want to get something granting +levels to Static and most likely Brimstone as well, over a gun that just grants you lightning damage), or maybe a Savagery Trickster, which needs dual wield melee to work anyway so the gun won’t work for it.
Copaine, you have any good BWC Fire Sorc gear?
I believe i have a Exon in my bank that i’d be willing to part with granting you got a legendary of same value
And btw, it is self-found and i got it a few days ago in Bastion of Chaos.