Best best is an MT from a Z31 - the N/A models had the same engine as the M30 and the same auto trans, so the manual ought to fit. The tricky part is the driveshaft - you'll either have to hope that either the stock one can be cajoled into working, the Z31 MT shaft works, or go the custom route (which would cost a bit). I'm going to look into this over spring break, hopefully, as the M30 really does need a manual.
BTW: hit
http://www.phpbbforfree.com/forums/?mforum=jcrapps for more M30 info.