I measured an O-ring out of my spare parts box today and here's what I got.
Metric O-rings are measured/specified/classified by the Inside Diameter (ID) and the Cross Section (CS).
The O-ring from my kit is 2.0 mm CS and 8.5 mm ID.
The part number is therefore M2x8.5.
The Outside Diameter (while not specified) is the 8.5 mm + 4.0 mm or 12.00-12.20 mm.
I found a wholesale distributor online with a very nice chart of metric O-Rings.
I guess you could call them and see who distributes their Metric products in your area.
Here's the link to the chart:
Hope this helps,