You can find it at the Sacred Ground Ruins (refer to the images for the precise location.)
It should respawn, but I didn't test it.
Make sure to enable "Estimate RSTB values for merged files" in the BCML
options (cogwheel icon in the lower right corner)The rapier has 150 durability and 34 damage.
Also, if you use either the Zelda or Linkle mods, make sure to give this mod lower priority.