I have a DS213j - very slow machine. You can pick it up from free if you want it. I haven't used it for a while so don't know if it still works.
Otherwise I'm also selling a compact 4 bay NAS case with power supply for 180$. You'll need to add a motherboard/cpu/ram to it and any NAS software like opednmediavalut, trueNAS or Unraid.