Alternately they are rebuildable.
There are companies that sell a full rebuild kit in the US but I've heard mixed things about the quality of some of the kit parts. This kit should be good as it uses all Japanese bearings etc.:
https://zilvia.net/f/showthread.php?t=417958
True Nissan OEM parts for most or all of the bearings, bushings, synchros, etc. still exist but you have to dig into the parts diagrams and order them from a place like partsouq or amayama. Nissan USA doesn't stock them but they're still available in Japan and UAE to ship to the US.
Once you have everything then any transmission shop can rebuild it.