Prior to Generation 9, single-use held items used outside of Battle Facilities/Link Battles are consumed permanently. Rejuv is currently based on gen 8.
The gen 9 mod for Rejuv has an option for whether you want to use Gen 8 or Gen 9's mechanics to this regard, and as Gen 9 content is officially coming in V14, this will likely be applied after the update as well, though may not be or come with a change in when certain consumable held items are first accessible for balancing.
That's Reborn. This was posted in the Rejuv part of the forum.
Currently there are only 3 Focus Sashes obtainable in Rejuvenation (1 in Blacksteeple, 1 in Chapter 13, and 1 in the Karma Files.).
Going off of what's in the Obsidia Department Store and Somniam Mall though, buying Focus Sashes should be possible in V14.
The current version of the game is V13.5.7. This was fixed in V13.5.1. Run the updater, it also fixes a bug that causes a battle to crash your game considerably later on.