I haven't seen anyone just doing the metric rings. Oring monkey sells the kit with all of them, or you can go to one of several oring retailers online and order them that way. I've had trouble finding the metrics at the hardware store...most I've been to only go down to 1.5 thickness, not the 1 thickness.
cool! I'll give the o-ring monkey kits a try since it includes all the metric o-rings. thanks!