Are those well nuts? If so, try either McMaster-Carr or Grainger. Especially McMaster-Carr, they have a huge selection of hardware, plus they give precise measurements of the hardware they sell. Makes it easier to choose the right parts.
Check your local True Value or Do It Best hardware store. If they carry bins of single nuts/bolts and assorted hardware you might get lucky. Metric will be the challenge. You can also go to your favorite dealer or on-line retailer and order OEM.