SJS Cycles or Spa Cycles - drop them an email. If you can measure the spoke (google on line), and assuming it's a standard J-bend one, these are plentiful. No guesses it will be a DT Swiss spoke, but you are likely to need to order a minimum of 6 (they come on a box/packet), although some may sell them individually. Remember, most rear spokes are a slight different length, drive side being shorter.
If your measurement is out by a couple of MM, this won't matter too much.