The short section of rail from about the middle of the cabin up to the anti roll bar is available from Nissan but in short supply, or at least thats what I was told when I ordered mine a few years ago. They were on back order for about 3 months which isn't to bad I suppose. From memory I think they were about £30 - 40 each.
The job to replace them is a long winded pig of a job. The OEM ones are spot welded onto the shell, so getting to old ones off isn't to bad, you just need to drill out the old spot welds and rip off the old mangled rails.
To get the new ones on is the tricky bit, unless you have a spot welder with deep arms about 2ft long.
Under the carpets and stuck to the floor are bitumen sound deadening panels, which need to chipped off and the underseal on the underside the underseal needs to be stripped off before any welding. Thats the messy bit. The tricky bit is aligning the new rails and tacking them into position. The positioning is critical, because the anti roll bar bolts up onto them, so if you get the position wrong you wont be able to bolt up the ARB. I used panel pins to hold them in position and bolted the ARB to them, then tacked them on, then removed the pins before seam welding the new rails on to the shell.
Safety first though, one of the most important things is to be able to get proper access to the underside of the car so make sure that once the car is up off the ground that it stable and secure.
