Does McCandless find happiness in Into the Wild?

It is not possible to know if McCandless ever found true happiness or not.  We would have to ask him, and then hope that he told the truth.  Based solely on how he died, no, I don't think that he found happiness.  From Krakauer's account, it's clear that McCandless was panicking near the end of his life.  

S.O.S. I NEED YOUR HELP. I AM INJURED, NEAR DEATH, AND TOO WEAK TO HIKE OUT OF...

It is not possible to know if McCandless ever found true happiness or not.  We would have to ask him, and then hope that he told the truth.  Based solely on how he died, no, I don't think that he found happiness.  From Krakauer's account, it's clear that McCandless was panicking near the end of his life.  

S.O.S. I NEED YOUR HELP. I AM INJURED, NEAR DEATH, AND TOO WEAK TO HIKE OUT OF HERE I AM ALL ALONE, THIS IS NO JOKE. IN THE NAME OF GOD, PLEASE REMAIN TO SAVE ME. I AM OUT COLLECTING BERRIES CLOSE BY AND SHALL RETURN THIS EVENING. THANK YOU, CHRIS MCCANDLESS. AUGUST?

McCandless was starving to death and too hurt to do anything about it.  No, I don't think he was happy when he wrote that note.  

But in the months preceding his death, yes, I think that McCandless had found happiness.  For McCandless being on the road, being accountable only to himself, and proving his self sufficiency was all that he wanted and needed.  He did that for months at a time.  He went where he wanted, and he went when he wanted.  He experienced a life out in the wilderness like his idol Jack London so frequently wrote about.  Additionally McCandless lived a sort of ascetic lifestyle like another one of his idols -- Tolstoy.  Yes, I think McCandless was happy.  And I think he was thrilled with his life for the few weeks that happened after Jim Gallien dropped him off in the Alaskan wilderness.  It was his dream to do, and he was finally going to live it. 

McCandless had been infatuated with London since childhood. London’s fervent condemnation of capitalist society, his glorification of the primordial world, his championing of the great unwashed—all of it mirrored McCandless’s passions. Mesmerized by London’s turgid portrayal of life in Alaska and the Yukon, McCandless read and reread The Call of the Wild, White Fang, “To Build a Fire,” “An Odyssey of the North,” “The Wit of Porportuk.”