I think Ebay had some for a good price recently? Otherwise I'm not sure, but they're pretty cheap from what I've seen! Got some in the garage 'just in case'
I can't remember where I got mine from, but Beevers Auto Panels are a good source. Although not genuine Nissan parts, they are very reasonable. Choice of outer sill only or also the inner sill, which will be knackered also.
I cut from the unused outer sill and used that material to re-weld the inner sill and re-work the wheel arch join in a more desirable manor......depends on how much you are replacing if you'll have enough material left.....still it's dirt cheap none the less