The P10 touring transmission is rather rare (only made 1994-96), but you can still find them around. You can find them in salvage yards (check
http://www.car-part.com for example) or maybe somewhere like
http://www.JGYCustoms.com The easiest way to check is have them read you the transmission code. If it ends in V it's vlsd equipped. If it ends in A, then it's an open diff transmission.
Heavy Duty, you can be amazed, the regular G20's were an open diff transmission (other than a select few early model Canadian P10's).
As far as an SE-R transmission they are much more common, with the VLSD. You can get one out of 1991-1994 Sentra SE-R, NX2000, or a 1995-1997 200sx SE-R. You will have to swap the speedo gear on the earlier models as it's a mechanical sender on that transmission, but it's an easy swap. And then for the halfshafts you'll have to run an automatic P10 halfshaft on the driver's side, from what I remember. (there's more info about this over at G20.net)
The SE-R transmission has shorter 1st, 3rd, 4th, and 5th gears, they kept 2nd the same in both (probalby to keep the 0-60 runs to only one gear shift in all the cars).
Or you could buy any P10 transmission and have the gear sets swapped over, if that's where the problem is at?