Try either a motorcycle dealer or see if you have a Grainger in your area. Here in Detroit, we have several fastener houses where we can buy damn near anything. I can usually find what I need at Ace Hardware though. For Datsun fasteners though, I have a stash that stems from nearly 30 years of parting cars out. I have them all sorted by type and size in plastic cases.

If all else fails, get them from McMaster-Carr. You can find them on the interwebs.

Are you sure you don't need the 8mm screw? Could you grab a drill bit or pencil or something to see which one is the same size as the hole?

They are M6x1.25. Any Z fastener with a 10mm head is an M6. The 12mm head fasteners are usually M8