If you want a quality engine contact Karim (racer98).
If you want the N1: http://cgi.ebay.com/ebaymotors/RB26DETT-R34-GTR-N1-Nur-Spec-Engine-Unit-longblock_W0QQcmdZViewItemQQ_trksidZp3286Q2em20Q2el1116QQhashZitem22f9b135e7QQitemZ150218028519QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
If you have the money for the N1, why not go ahead and build a new one?
There's an N1 short block on ebay for $1,800
Nismoparts.com has the crankshaft for $577
From there you have room for the intake, head, and you can go forged on the rest of the bottom end. I know it will end upt costing more, but you will have a built bottom end.
Or one more option, better yet than the N1 RB26DETT. If you have the money to spend, contact Rob at RIPZ. Go for the RB30DET