This is where I went for mine a few months ago. He has them in stock and can ship them out immediately.
Installation instructions can be found on Sentra.net and it's pretty simple. The most difficult part would be the wiring but even that is easy if you've done stuff like installing you own stereo or modified your foglights. It really isn't that hard to do, just make sure you take your time.
On a '91-'92, this headlight improves night time vision dramatically since you are switching from a 9004 bulb with a refractive lens cover to an H4 with a clear headlight cover using a multi-angle reflector. On the '93-'94, I'm sure it is still an improvement but not as much as the earlier model.