Sensi Seeds all the way, as they are keepers of the original.
Although it does take a good amount of pheno hunting to find a keeper.
I know, cause I bought a 10 pack from Sensi and I still have 3 Jack seeds left to find my keeper.
Sensi's Jack is not the same as it used to be so I don't think it's the original but rather a new selection maybe due to the loss of one of there parents.
I grew some Jack Herer from Sensi Seeds in the early nineties. The plants were out of control and the bag appeal wasn't that great, but the yield and the potency almost made it worth dealing with them.
That would be my first choice too if I was after a pure Jack and not a X, I went with Jack Hammer & JackBerry as it's high time I had some JackX's in my garden. I went with Sannie after seeing all the great grow reports of his gear around the forums.